Enable unused var warnings under typing/

Summary: Revealed a number of minor bugs (e.g. expanded types not
getting used, env being discarded after being modified). I'm also hoping
this will make it easier to spot unused fields in the records that we
store in shared memory.

Reviewed By: @elgenie

Differential Revision: D1758813

Signature: t1:1758813:1419990490:5b4133f8d239d946cf800b311176e8301d6f0dff