Add poll to FIleOperations

[alex]

(Split out from #164)

There's a few moving pieces here:

• Adding the actual trait and setting up the function pointer
• Figuring out a safe API for poll_wait -- the lifetimes are pretty tricky, might be best to start with just the &'static case.
• This may require adding a basic wait queue API at the same time, so we have some way to test this!