My slab bump in #246 added a potential new panic in Core's Inner's
`cancel_timeout`.
Thankfully, the only way `cancel_timeout` can be called is from
TimeoutToken's `cancel_timeout`, which is crate-internal only and is
only called from Timeout's and Interval's drop fn's.
This change simply adds new clarifying documentation around
TokenTimeout's cancel_timeout to "future proof" anybody looking to use
cancel_timeout directly (not just on drop).
* 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