nixpkgs: Add cryptol2 expression

This is just a convenient shorthand so people don't have to spell out
haskellPackages.cryptol

Note that the top-level expression is named 'cryptol2' but the package
isn't. That's because Cryptol is a library and other things could depend
on it (hence the vanilla name), but also the full name will be
disambiguated as 'haskell-cryptol-ghc7.6.3' anyway.

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