windows: Introduce `MessageReader.fetch_async_result()` helper method
windows: Introduce `MessageReader.fetch_async_result()` helper method

Move the raw FFI call from `OsIpcReceiver.get_message()` to a new helper
method in `MessageReader`.

This fixes a major layering violation, and makes the functionality more
reusable.

Note: this doesn't address the related raw FFI call in
`OsIpcReceiverSet.select()` -- that one is a whole other can of worms,
which will require a larger refactoring to fix...

servo/ipc-channel