Stamp out space leaks from demand analysis
This reduces peak memory usage by ~30% on my test case (DynFlags),
and (probably as a result of reduced GC work) decreases compilation
time by a few percent as well.
Also fix a bug in seqStrDmd so that demeand info is fully evaluated.
Reviewers: simonpj, austin, bgamari
Reviewed By: bgamari
Subscribers: dfeuer, thomie
Differential Revision: https://phabricator.haskell.org/D3400