CVC5 support via an indirection through kd.smt module.

programmatically setting before importing knuckledragger.

import os
os.environ["KNUCKLE_SOLVER] = "cvc5"
import knuckledragger as kd

or

KNUCKLE_SOLVER=cvc5 pytest

It is not clear if this is a good approach, or just deal with cvc5 the way we will have to deal with vampire and others.

Currently there are some issues about Lambdas I don't know how to deal with cvc5/cvc5_pythonic_api#100