Introduce {Fixed,Advice,Instance}Query for Expression::{Fixed,Advice,Instance}

[han0110]

This PR aims to make Expression::{Fixed,Advice,Instance} wrap their own *Query structure, to make it possible to add private fields in each variant (e.g. #593 needs to add a private field phase).

For backwards compatibility (to make vk to hash to same value), it implements fmt::Debug for Expression manually.

Also to make module dev less difference, it also introduces non-public structure AnyQuery to reuse existing helper functions.

[str4d]

utACK. This is all so much cleaner now, I love it!

[str4d]

ACK 6987b2e. I checked that this revision still passes orchard tests, including the serialized proof test case (ensuring that the Orchard circuit is unaltered by this change).

[therealyingtong]

utACK. #633 should be rebased on this.

[daira]

Post-hoc ACK