Adjust feature gates to allow for parenthetical notation to be used
with the fn traits
Mostly rote conversion of `proc()` to `move||` (and occasionally `Thunk::new`)
Delete tests that seem to serve no purpose anymore.
More `proc()` -> `move||` changes.
Update guide/intro to take into account the removal of `proc`.

cc @steveklabnick
Update emacs and vi modes.
Remove `proc` types/expressions from the parser, compiler, and
language. Recommend `move||` instead.
Rewrite WorkItem not to use `proc()`.
Rewrite threading infrastructure, introducing `Thunk` to represent
boxed `FnOnce` closures.
Patch rustdoc to include missing types, make the match exhaustive
to prevent such oversights in the future.
WIP -- FIXME needed -- debuginfo test that doesn't work and I don't know why