Implement async I/O support in System.IO.FileSystem/Pipes on Unix

[stephentoub]

System.IO.FileSystem has an initial implementation on Unix, but it doesn't yet have special support for async I/O (right now the async operations just rely on the base Stream type's implementation, which will schedule a work item to invoke the synchronous implementations).

[stephentoub]

After further investigation, there doesn't seem to be much more we can do here for FileStream. io_submit would require restrictions on the FileStream (e.g. requiring O_DIRECT) and is only available on Linux. epoll doesn't work with files. And aio would essentially provide the same solution we already employ, offloading the synchronous call to another thread.

[jonmill]

Okay cool, if this isn't actionable anymore then I'll close the issue. Thanks Stephen!