Enable pattern matching warnings

A quick fix for #144, though it negates the performance benefit of a fast path for non-TH code.  According to the [GHC module documentation](http://www.haskell.org/ghc/docs/latest/html/libraries/ghc/GHC.html#t:HscTarget), while setting HscTarget to `HscNothing` skips code generation, it also skips desugaring, which is when pattern matching warnings are detected.

GHC docs also mention that desugaring can be run standalone, which I will try implementing/benchmarking later this week.