Changes for next breaking change release

[ebfull]

There are some things we may change in halo2 to maintain backwards compatibility with proofs that were constructed in older versions of halo2. However, once we do actually make a breaking change we want to remove the reverse compatibility code to reduce technical debt.

Let's maintain a list of things like that:

• Introduce {Fixed,Advice,Instance}Query for Expression::{Fixed,Advice,Instance} #616 maintains reverse compatibility for Expression debug formatting in order to keep the verification key hash the same as before.

[therealyingtong]

Since we only permit fixed queries at Rotation::cur(), there is no longer any need to track fixed_queries rotations in the constraint system. For now, these are kept for backwards compatibility. #633