Allowing the use of tokio's AsyncBufRead

[garma83]

Supporting async buffer streams is a major missing feature I think. In the past there has been quite a bit of discussion about it and it looks even like it has mostly been finished. It might even just be waiting for a merge. Would be great if this can be rescued

tafia/quick-xml#314

[Mingun]

PR is welcome! As noticed in the tafia/quick-xml#314, I would prefer to reuse a existing code rather than duplicate it -- or using macros, or introduce a new traits / methods in the existing traits.

Also, it seems there are more than one implementation of async readers -- at least async-std also wide used. Reducing duplication in the code would help to support it as well.

The dependence on tokio looks a little overkill -- it's a pretty big crate. Is it possible to depend on the crate, which provides only a trait (is that crate exists)?