Implement POSIX FD abstraction

[acw]

POSIX is deeply annoying for any number of reasons; one of them is that it mixes up "file descriptors" between files and network sockets, because blah blah blah.

What this means for the HaLVM is that if we want to have a chance at supporting anything like normal C libraries, we're going to have to have a common layer inside the HaLVM that dispatches based on the exact FD type.