draft Buf writer

[Drevoed]

This pull request will resolve #83 when merged.

It includes asynchronous BufWriter and LineWriter with tests.
Currently I'm having problems with implementing poll_write for BufWriter, because trying to call
self.buf().write(buf).poll(cx) results in

error[E0599]: no method named `poll` found for type `io::write::WriteFuture<'_, std::vec::Vec<u8>>` in the current scope
   --> src/io/buf_writer.rs:239:33
    |
239 |             self.buf().write(buf).poll(cx)
    |                                 ^^^^
    |
   ::: src/io/write.rs:149:1
    |
149 | pub struct WriteFuture<'a, T: Unpin + ?Sized> {
    | --------------------------------------------- method `poll` not found for this
    |
    = help: items from traits can only be used if the trait is implemented and in scope
    = note: the following traits define an item `poll`, perhaps you need to implement one of them:
            candidate #1: `core::future::future::Future`
            candidate #2: `futures::future::Future`
            candidate #3: `futures::stream::Stream`

I assume that's because WriteFuture has trait bounds on ?Sized but I'm not sure how to get around that.

The above error was fixed by wrapping the buf in Pin explicitly.

Previous PR was closed because I forgot to mark it as draft.

[skade]

I think this PR could use some tests, especially around how BufWriter is used.

[ghost]

Can we perhaps move LineWriter into a new pull request to keep this one simple?

[yoshuawuyts]

Ref #131 also (tracking issue for async_std::io)

[yoshuawuyts]

@Drevoed o/ wanted to check in how things are. Is there anything we can do to help you get unblocked?

[Drevoed]

@yoshuawuyts I'm currently a bit busy with day-time job but will try to implement requested changes ASAP, I don't think I will have any problems, but if I will - I am going to ask for assistance surely :)

changes resolved

[yoshuawuyts]

This patch looks excellent to me! -- I think we should go ahead and merge this in. Thanks so much for all the work you've put into this; this is really good!