In-process simulator backend

[yupferris]

Mostly to simplify testing, as such a simulator would likely be at least an order of magnitude slower than generated Rust code, though it would probably be much more convenient to use, especially for smaller modules and quick prototypes/tests.

My current thinking is that this would be similar to Rust sim gen with module hierarchy flattening, but could generate custom commands that would either be interpreted (simpler) or JIT'd (faster, more fun).