Support await syntax for async methods

[Boddlnagg]

This should be possible with Rust's coroutines support, see https://doc.rust-lang.org/nightly/unstable-book/language-features/generators.html and rust-lang/rust#43122.

We might need to provide our own macros similar to https://github.com/alexcrichton/futures-await.

[Boddlnagg]

I got a proof of concept running in a branch: I started working on this in a branch: https://github.com/Boddlnagg/winrt-rust/tree/futures-preview

I must say that it feels quite nice!