Add T7220a.stderr

which presumably was just forgotten when creating the testcase in
commit 7b1a856.
Update Control.Monad.ST.* for Safe Haskell as now they're safe by default
Update Foreign.* for Safe Haskell now that they're safe by default
Improve Safe Haskell bounds for changes to base over time
Be consistent with placement of Safe Haskell mode at top of file
Implement Partial Type Signatures

Summary:
Add support for Partial Type Signatures, i.e. holes in types, see:
https://ghc.haskell.org/trac/ghc/wiki/PartialTypeSignatures

Test Plan: validate

Reviewers: simonpj, goldfire, austin

Subscribers: simonmar, ezyang, carter, goldfire

Differential Revision: https://phabricator.haskell.org/D168