Allow richer Python values for property arguments

[acfoltzer]

Currently, _CryptolModule.{check, exhaust, sat, prove} all take a string argument representing the property to check. It should be possible to pass in Python values to these functions, as long as those Python values originated from Cryptol in the first place. So if you have:

module Foo where

bar : [32] -> Bit
property bar x = x == x

On the Python side you ought to be able to have something like:

Foo = Cryptol().load_module("Foo.cry")
Foo.prove(Foo.bar)

[acfoltzer]

We'll need to keep track of some syntactic representation of the value for this to work.