While most of cppcoro is platform agnostic, some of the thread-pooling and I/O code is OS-specific.

This issue is for providing an implemention of the cppcoro I/O abstractions for Linux.

Will need to get it building under Clang as a first step. See #3.
Then:

• Look into writing an epoll I/O event loop, based io_service implementation.
• Port file and socket (once written) interfaces to Linux.
• Add build-system support for Linux