Proof of Concept io-uring libraries, work in progress

See https://twitter.com/axboe/status/1114568873800491009 for what io-uring is about.

Also see "echo-server" example.

Crates:

• io-uring-sys: Low-level wrapping of the kernel API and types
• io-uring: Fancy wrapping of kernel API (especially submission and completion queue)
• tokio-uring-reactor: Reactor (IO handling) based on io-uring for tokio integration.
• tokio-uring: tokio (current_thread) Runtime based on io-uring.

Right now some very basic TCP (accept, read, write) operations are supported by the tokio integration directly, but you can do almost anything using the async_* functions provided by the reactor handle.