Mutable keys are a minefield for comparisons, as resolving
equality require re-implementing a lot of the internal structures, as
well as dealing with multiple mutable keys that are in the same
equivalency class by deep=.
Simplifying the implementation to not resole mutable keys is much
simpler, faster, and has the benefit that deep= and deep-not= do not
need to allocate.
Calvin Rose
746ced5501