Remove defunct stuff, rewrite test harness.

Now it just uses QuickCheck so it's far more portable, and it should
build faster. Because of this, I also expanded the range of GHC versions
in travis again.

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Rename C functions: crypto_* -> ed25519_*

This will make them less likely to clash if someone links to a separate
sodium/nacl binding.

Signed-off-by: Austin Seipp <aseipp@pobox.com>