explicit instantiation of common Future types

explicit instantiation of common Future types

Summary:
Compiling folly/futures:futures-test, this saves 15% (dbg) and 7% (opt)
Compiling all of folly/futures, it's 7% ish for each

Main blocker right now is that this generates a spew of deprecated warnings from calls to core_->(de)activate() from Future::(de)activate(). Can the deprecations be moved up to the Future methods instead?

Also had to fix willEqual for Future<void> which was borked

Test Plan: compiles

Reviewed By: hans@fb.com

Subscribers: trunkagent, folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D2021028

Signature: t1:2021028:1430749114:1dd78af47ea91aa5e67929a5884b66ca0c8ae2d8