Revert "Disable caching of imports (fixes #215), and make sure stateImported is never cleared"

This reverts commit f4083aaf17a0ff821bcbc00cdddcdf268565b568.

Conflicts:
	src/Language/Fay/Compiler.hs
	src/Language/Fay/Types.hs
Add reproducing test case for issue #215
Reimplement caching correctly for #215 and #223
Add back some comments lost in the revert
Remove unneeded CompileError constructor