Clean up stale Haskell

[thumphries]

• If you delete a template, the rendered Haskell lingers and is picked up by GHCi
• If you move a template, that will also cause problems with GHCi

Loom is supposed to do the right thing, so it should handle this scenario without resorting to something like rm dist/loom or loom clean. Stale generated files should be detected and removed.