Rust bindings to Zephyr networking

[d3zd3z]

Zephyr's network stack tries to present a posix-inspired interface to networking. At the time of
writing, there is a draft PR that adds some initial support for interfacing directly to this.

This interface is largely blocking. There is a poll API in Zephyr, and using this for async would
be similar to what is needed to add async support to Semaphores. However, triggered work does not
currently support polling sockets, so this would need to add either a separate thread, or could dive
under the hood a bit, and leverage the semaphores used by the Zephyr networking stack.