Unit tests
Make 'fail' report an error in 'ParseResult' (refs #4)
Use fixities in parseStmt* variants (refs #50)
Add Data / Typeable to Language / Extension
Support only GHC 7.4+

Since HSE has limited development resources, it's better to have tested and
working code for modern compilers than do a poor job of supporting all possible
implementations.

If you do need support for a non-GHC compiler or an older version of GHC, file
an issue and explain why you need it. We'll work something out.
multi-ghc travis