nixpkgs: Add cryptol2 expression
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 <48c905ac3851f382b914247d97dd93273d5ff555@pobox.com>

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