Promise::setValue() for Unit
Summary: Unit is a bit special because it's just something special to use instead of `Promise<void>`, so let's offer the same sugar that `Promise<void>` has (`p.setValue()` instead of `p.setValue(Unit())`)
Test Plan: New unit tests. Look, a pun!
Reviewed By: jsedgwick@fb.com
Subscribers: exa, folly-diffs@, jsedgwick, yfeldblum, chalfant, davejwatson, hannesr
FB internal diff:
D2014139
Tasks:
6847876
Signature: t1:
2014139:
1430159950:
1484ee420c6d7f0f794a546b78ef1601c2eec45c