add async <--> blocking "bridge" Fns to async channel

[MikeIvanichev]

Adding blocking methods to the async mpsc channel

Addresses: #26

This PR aims to implement a bridge between blocking and sync code by adding blocking methods
to the async mpsc::{Sender, Receiver}.

The implementation follows the suggestions in #26, wrapping a Thread in a Waker when using a blocking method.

Progress

All things marked as completed have been implemented and documented.

• Impl tests
• Impl loom tests
• Loom testing (2 preemptions)
• Impl benchmarks

mpsc::Sender

• blocking_send_ref
• blocking_send_ref_timeout
• blocking_send
• blocking_send_timeout

mpsc::Receiver

• blocking_recv_ref
• blocking_recv_ref_timeout
• blocking_recv
• blocking_recv_timeout

mpsc::StaticSender

• blocking_send_ref
• blocking_send_ref_timeout
• blocking_send
• blocking_send_timeout

mpsc::StaticReceiver

• blocking_recv_ref
• blocking_recv_ref_timeout
• blocking_recv
• blocking_recv_timeout

[MikeIvanichev]

Hi @hawkw,
Though this isn't done yet, I wanted to get your opinion on the direction I'm going earlier rather than later.

Please let me know if there are things I could do better, or that you would prefer be implemented differently.