add RtioPoll for waiting for a file descriptor to become ready

This commit adds a new trait `RtioPoll` to `std::rt::rtio`.  An `RtioPoll`
wraps a file descriptor and provides a single method `await` which blocks the
calling thread until the file descriptor becomes ready (for reading, for
writing, or for either, depending on the argument to `await`).  This will be
useful when creating wrappers around native C libraries that expect the client
code to `select` on a particular file descriptor and then (once the file
descriptor is ready) call a library function to process events.

Currently, `RtioPoll` is implemented only for `libgreen` - attempting to
construct one using a `libnative` `IoFactory` returns an error.  The `libgreen`
version (actually located in `librustuv`) is implemented on top of  `libuv`'s
`uv_poll_t` handle type.