Add a crate that provides a test suit for an implementation of StreamMuxer

[tomaka]

Add a crate that provides:

pub fn test_muxer<T: StreamMuxer>(muxer: T) { ... }

Of course the implementation would check whether the muxer behaves as expected.

Then mplex, yamux and future implementations could import that crate and run the function.

[tomaka]

We should do #493 first

[tomaka]

cc @dvdplm