Unbundle/rewrite mpz-garble

[sinui0]

The mpz-garble crate is a bit of a mess as it was implemented on a deadline. It needs to be rewritten and unbundled.

Some high-level steps:

1. Move all VM related abstractions into a dedicated crate.
2. Decouple the Generator and Evaluator types from the VM abstractions. They should have simple APIs which do not operate on a memory type but simply execute a circuit with provided input encodings.
3. Completely rewrite the VM memory implementation to be backed by linear memory and fix the clunky type representation model.

[sinui0]

#191