Handle functions in nested data

[acfoltzer]

The code for building a closure on the Python side out of a Cryptol value currently only checks the top-level type of the expression to see whether it's a function. This means that if a function is wrapped up in, say, a tuple, it does not get converted into a callable Python function.

Unfortunately this will require a fair amount of refactoring on the cryptol-server side given how we're generating JSON.