socket2 0.3.19

Utilities for handling networking sockets with a maximal amount of configuration possible intended.
Documentation
# Refactor

* [ ] CONTRIBUTING.md
* [ ] Move tests from src/*.rs to tests dir.
* [ ] Tracking issue for tests. Check all functions/coverage (with `rustcov`).
* [ ] Look at `SockAddr`.
  * [ ] Add docs about its size.

## Maybe add socket options

# SOL_SOCKET
SO_ACCEPTCONN
SO_DOMAIN
SO_TYPE
SO_INCOMING_CPU // Linux only?
SO_INCOMING_NAPI_ID // Linux only?

# No-op/error returned on Windows:
SO_RCVLOWAT
SO_SNDLOWAT