More typechecking for ghost variables

[langston-barrett]

Fixes #548 and #543

Here's what it looks like on test_ghost_branch_03:

[16:59:17.110] Subgoal failed: h safety assertion: Attempted to mux ghost variables of different types:
Bit
[32]

And in the other case:

  Abort due to false assumption:
    Ghost variable had the wrong type:
    - Expected: [75][8]
    - Actual:   [45][8]
    in _ZN5folly3ssl11OpenSSLHash4Hmac10hash_finalENS_5RangeIPhEE at :123:30

[langston-barrett]

This hasn't yet been tested on Fryingpan/Hydra, which is experiencing some evaluation errors: http://fryingpan.dev.galois.com/hydra/jobset/s2n/PR-typecheck-ghost.submodules-ghc865

[langston-barrett]

This has passed on Fryingpan: http://fryingpan.dev.galois.com/hydra/build/233554

[brianhuffman]

Looks good.