Compilation warnings with Clang

[shreyasbharath]

A whole bunch of warnings pop up when compiling with Clang.

Can we fix these please?

picosha2.h:199:57: error: implicit conversion changes signedness: 'std::size_t' (aka 'unsigned int') to 'std::__1::__wrap_iter<unsigned char *>::difference_type' (aka 'int') [-Werror,-Wsign-conversion]
            detail::hash256_block(h_, buffer_.begin() + i,
                                                      ~ ^
picosha2.h:200:53: error: implicit conversion changes signedness: 'std::size_t' (aka 'unsigned int') to 'std::__1::__wrap_iter<unsigned char *>::difference_type' (aka 'int') [-Werror,-Wsign-conversion]
                                  buffer_.begin() + i + 64);
                                                  ~ ^
picosha2.h:202:58: error: implicit conversion changes signedness: 'std::size_t' (aka 'unsigned int') to 'std::__1::__wrap_iter<unsigned char *>::difference_type' (aka 'int') [-Werror,-Wsign-conversion]
        buffer_.erase(buffer_.begin(), buffer_.begin() + i);
                                                       ~ ^
picosha2.h:301:32: error: implicit conversion changes signedness: 'int' to 'std::__1::vector<unsigned char, std::__1::allocator<unsigned char> >::size_type' (aka 'unsigned int') [-Werror,-Wsign-conversion]
    std::vector<byte_t> buffer(buffer_size);
                        ~~~~~~ ^~~~~~~~~~~
picosha2.h:199:57: error: implicit conversion changes signedness: 'std::size_t' (aka 'unsigned int') to 'std::__1::__wrap_iter<unsigned char *>::difference_type' (aka 'int') [-Werror,-Wsign-conversion]
            detail::hash256_block(h_, buffer_.begin() + i,
                                                      ~ ^
picosha2.h:293:12: note: in instantiation of function template specialization 'picosha2::hash256_one_by_one::process<std::__1::__wrap_iter<const char *> >' requested here
    hasher.process(first, last);
           ^
picosha2.h:323:21: note: in instantiation of function template specialization 'picosha2::impl::hash256_impl<std::__1::__wrap_iter<const char *>, unsigned char *>' requested here
    picosha2::impl::hash256_impl(
                    ^
picosha2.h:346:5: note: in instantiation of function template specialization 'picosha2::hash256<std::__1::__wrap_iter<const char *>, unsigned char *>' requested here
    hash256(first, last, hashed, hashed + k_digest_size);
    ^
picosha2.h:360:5: note: in instantiation of function template specialization 'picosha2::hash256_hex_string<std::__1::__wrap_iter<const char *> >' requested here
    hash256_hex_string(src.begin(), src.end(), hex_str);
    ^
picosha2.h:200:53: error: implicit conversion changes signedness: 'std::size_t' (aka 'unsigned int') to 'std::__1::__wrap_iter<unsigned char *>::difference_type' (aka 'int') [-Werror,-Wsign-conversion]
                                  buffer_.begin() + i + 64);
                                                  ~ ^
picosha2.h:202:58: error: implicit conversion changes signedness: 'std::size_t' (aka 'unsigned int') to 'std::__1::__wrap_iter<unsigned char *>::difference_type' (aka 'int') [-Werror,-Wsign-conversion]
        buffer_.erase(buffer_.begin(), buffer_.begin() + i);
                                                       ~ ^
picosha2.h:98:59: error: implicit conversion changes signedness: 'unsigned int' to 'std::__1::__wrap_iter<unsigned char *>::difference_type' (aka 'int') [-Werror,-Wsign-conversion]
        w[i] = (static_cast<word_t>(mask_8bit(*(first + i * 4))) << 24) |
                                                      ~ ~~^~~
picosha2.h:199:21: note: in instantiation of function template specialization 'picosha2::detail::hash256_block<unsigned long *, std::__1::__wrap_iter<unsigned char *> >' requested here
            detail::hash256_block(h_, buffer_.begin() + i,
                    ^
picosha2.h:293:12: note: in instantiation of function template specialization 'picosha2::hash256_one_by_one::process<std::__1::__wrap_iter<const char *> >' requested here
    hasher.process(first, last);
           ^
picosha2.h:323:21: note: in instantiation of function template specialization 'picosha2::impl::hash256_impl<std::__1::__wrap_iter<const char *>, unsigned char *>' requested here
    picosha2::impl::hash256_impl(
                    ^
picosha2.h:346:5: note: in instantiation of function template specialization 'picosha2::hash256<std::__1::__wrap_iter<const char *>, unsigned char *>' requested here
    hash256(first, last, hashed, hashed + k_digest_size);
    ^
picosha2.h:360:5: note: in instantiation of function template specialization 'picosha2::hash256_hex_string<std::__1::__wrap_iter<const char *> >' requested here
    hash256_hex_string(src.begin(), src.end(), hex_str);
    ^
picosha2.h:99:59: error: implicit conversion changes signedness: 'unsigned int' to 'std::__1::__wrap_iter<unsigned char *>::difference_type' (aka 'int') [-Werror,-Wsign-conversion]
               (static_cast<word_t>(mask_8bit(*(first + i * 4 + 1))) << 16) |
                                                      ~ ~~^~~
picosha2.h:100:59: error: implicit conversion changes signedness: 'unsigned int' to 'std::__1::__wrap_iter<unsigned char *>::difference_type' (aka 'int') [-Werror,-Wsign-conversion]
               (static_cast<word_t>(mask_8bit(*(first + i * 4 + 2))) << 8) |
                                                      ~ ~~^~~
picosha2.h:101:59: error: implicit conversion changes signedness: 'unsigned int' to 'std::__1::__wrap_iter<unsigned char *>::difference_type' (aka 'int') [-Werror,-Wsign-conversion]
               (static_cast<word_t>(mask_8bit(*(first + i * 4 + 3))));
                                                      ~ ~~^~~
picosha2.h:199:57: error: implicit conversion changes signedness: 'std::size_t' (aka 'unsigned int') to 'std::__1::__wrap_iter<unsigned char *>::difference_type' (aka 'int') [-Werror,-Wsign-conversion]
            detail::hash256_block(h_, buffer_.begin() + i,
                                                      ~ ^
picosha2.h:293:12: note: in instantiation of function template specialization 'picosha2::hash256_one_by_one::process<std::__1::__wrap_iter<unsigned char *> >' requested here
    hasher.process(first, last);
           ^
picosha2.h:323:21: note: in instantiation of function template specialization 'picosha2::impl::hash256_impl<std::__1::__wrap_iter<unsigned char *>, std::__1::__wrap_iter<unsigned char *> >' requested here
    picosha2::impl::hash256_impl(
                    ^
blah.h:78:13: note: in instantiation of function template specialization 'picosha2::hash256<std::__1::__wrap_iter<unsigned char *>, std::__1::__wrap_iter<unsigned char *> >' requested here
  picosha2::hash256(image.begin(), image.end(), hash.begin(), hash.end());
            ^
picosha2.h:200:53: error: implicit conversion changes signedness: 'std::size_t' (aka 'unsigned int') to 'std::__1::__wrap_iter<unsigned char *>::difference_type' (aka 'int') [-Werror,-Wsign-conversion]
                                  buffer_.begin() + i + 64);
                                                  ~ ^
picosha2.h:202:58: error: implicit conversion changes signedness: 'std::size_t' (aka 'unsigned int') to 'std::__1::__wrap_iter<unsigned char *>::difference_type' (aka 'int') [-Werror,-Wsign-conversion]
        buffer_.erase(buffer_.begin(), buffer_.begin() + i);
                                                       ~ ^