Increase typechecking performance for Cryptol::Extras

[acfoltzer]

The definitions added in #299 wound up split out into the Cryptol::Extras module due to the performance regression they introduced (speed of typechecking the Prelude is of very high priority because it happens so often). Let's work on improving performance of the typechecker further, and then merge these definitions back into the regular Prelude. (cc @TomMD)

[TomMD]

The Cryptol::Extras module can not be imported without something like CRYPTOLPATH=$repo/lib. Is it easy to add the Extras module to wherever the prelude is installed so its in scope? Once that's done I'll add extras as an import to Minilock/* so it loads properly.

[TomMD]

@acfoltzer mentioned to me that the bindist should locate Cryptol::Extras fine and the issue is probably limited to cabal-install users. @elliottt, I think adding Extras.cry to Cryptol/Prelude.hs would fix the issue - thoughts?

[atomb]

Type checking Cryptol::Extras is now pretty quick. It seems to take about 0.05s on my machine (comparing the load time of cryptol with no arguments to running it on a file containing only import Cryptol::Extras).