For tests, we really don't need to do much more than build the compiler
and run the tests --- Cabal shenanigans shouldn't be really necessary,
so we can just use Stack here, which should handle caching much better.

The `builtin-arrays' builds are currently broken, except on Dargent
branches --- when those are merged, these builds should be flipped back
to required.

 * Cogent ext2fs builds on Travis.
 * BilbyFs doesn't currently compile; it really should...

While we're here, bump resolvers.

 * With +docgent, we use Pandoc, which won't Haddock in Stack.
 * With +haskell-backend, we get the haskell-lexer, which segfaults Haddock.