Make all non-test randomness user-provided

[sipa]

Instead of relying on the C++ std::random_device and std::uniform_int_distribution (which lack strong quality guarantees), move the responsibility for generating good randomness to the user.

All constructors and functions related to sketch construction are modified to take in an explicit uint64_t seed value. The test code still uses the C++ randomness functions.

Fixes #93.

[gmaxwell]

Maybe the API doc ought to continue to say that the seed is just to limit the ability of malicious input to intentionally cause worst case operating time? -- just so the caller doesn't worry to much about it and also so they know that they probably shouldn't let a third party pick the value.