[RPC] Update with latest cryptol_client, fix types

[m-yac]

This PR updates the SAW RPC Python API with the latest version of the Cryptol RPC Python API. Summarizing the most relevant change, what was written before as:

cryptol("(@)")(res, cryptol("0"))
cryptol("`{i}:[64]".format(i=length))
i < cryptol("512")

would now be written as:

cry_f("{res}@0")
cry_f("`{length}:[64]")
cry_f("{i} < 512")

This PR also adds precondition_f, postcondition_f, and returns_f, which are just shorthands for precondition(cry_f(...)), postcondition(cry_f(...)), and returns(cry_f(...)). For example, what was written before as:

self.precondition(i < cryptol("512"))

would now be written as:

self.precondition_f("{i} < 512")

Additionally, this PR updates various parts of the library and tests to get everything to typecheck with mypy, and makes some types more explicit where possible.

[pnwamk]

This PR is awesome. Just a few minor comments I'm happy with you addressing however you see fit. (Happy to chat more about tradeoffs too, but don't feel blocked by me over something trivial.)