In preparation for adding the zkEVM aggregation circuit. Mainly,
- Adds a `WitnessWrite` trait, a sub-trait of `Witness`, and move the write methods to it. `GeneratedValues` impls `WitnessWrite`, which lets generators like `DummyProofGenerator` access all our write methods like `set_proof_with_pis_target`. Also removes some duplication.
- Remove `set_cyclic_recursion_data_target` - now that dummy proof data is automatically populated, all that remains is populating `condition` and the cyclic proof + VK. I think it's easy enough for callers to do this; the steps are the same as with `conditionally_verify_proof`. This way there's no cyclic-recursion-specific API to learn about.
- Split `cyclic_recursion` into two variants, one which checks the current circuit or a dummy, and a more general one which checks the current circuit or some other circuit. We can use the latter to build a more efficient aggregation circuit, where we check another aggregation proof or an EVM proof, with no dummy proofs involved.
These appear to be unused for extension fields, so we're free to change the mapping without breaking anything.
As the TODO says, the mapping that's currently implemented doesn't seem natural or useful. It seems more natural to treat the `BigUint` as a base field element, potentially in a non-canonical form.
It seems redundant in most contexts, e.g. `use plonky2::field::extension_field::Extendable;`. One could import `extension_field`, but it's not that common in Rust, and `field::extension` is now about as short.
