chore: Remove dep on derivative (#7133)
(This is part of #5711)
## Summary
@BurntSushi and I spotted that the `derivative` crate is only used for
one enum in the entire codebase — however, it's a proc macro, and we pay
for the cost of (re)compiling it in many different contexts.
This replaces it with a private `Inner` core which uses the regular std
derive macros — inlining and optimizations should make this equivalent
to the other implementation, and not too hard to maintain hopefully
(versus a manual impl of `PartialEq` and `Hash` which have to be kept in
sync.)
## Test Plan
Trust CI?
