* Handle -> Remote
* Pinned -> Handle
All APIs now take a `&Handle` by default and in general can return an immediate
`io::Result` instead of an `IoFuture`. This reflects how most usage will likely
be done through handles rather than remotes, and also all previous functionality
can be recovered with a `oneshot` plus `Remote::spawn`.
Closes#15
We know that the future will never persist beyond this stack frame, so we can
just leave its ownership on the stack frame itself and receive notifications off
the event loop that we need to poll it.