util: add futures-io/tokio::io compatibility layer (#2117)

* util: add futures-io/tokio::io compatibility layer

This PR adds a compatibility layer with conversions between the
`tokio::io` and `futures-io` versions of the `AsyncRead` and
`AsyncWrite` traits.

I initially opened this PR against `tokio-compat`, but we decided that
a compatibility layer for current versions of the `tokio` and
`futures-io` crates (rather than for compatibility with legacy code)
ought to go in `tokio-util` instead. See:
https://github.com/tokio-rs/tokio-compat/pull/2#issuecomment-551310953

This is based on code originally written by @Nemo157 as part of the
`futures-tokio-compat` crate, and is contributed on behalf of the
original author:
https://github.com/Nemo157/futures-tokio-compat/issues/2#issuecomment-544118866

Closes tokio-rs/tokio-compat#2

Co-authored-by: Wim Looman <[email protected]>
Signed-off-by: Eliza Weisman <[email protected]>

tokio-util/src/lib.rs

@@ -25,3 +25,7 @@ cfg_codec! {
 cfg_udp! {
     pub mod udp;
 }
+
+cfg_compat! {
+    pub mod compat;
+}