Remove unsafe perform io

[adrianmay]

This is JP's #84 which removes most uses of unsafePerformIO and changes the Haskell interface to IO, plus:

• Merges from current master
• Adaptation of subsequently created benchmark code to the IOish interface
• A mutex for fec_init that's also free of unsafePerformIO
• A major version bump
• Warnings fixed
• Cabal version constraints reduced to hackage rules only

This PR (or some variation of it) is essential because unsafePerformIO is extremely unsafe. It may stand alone, but I recommend that it be immediately followed by #131 that completely removes fec_init and the Haskell wrapper's initialize, initialising the tables with verbatim values in C, and bumping the major version again.

Bumping the major version twice in quick succession strikes me as less bad than any other option. I wouldn't expect the intermediate version to get used but its existence mitigates any risk in the second PR.