draft Add BufWriter

[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.

[Drevoed]

r? @skade