[FR] Unified hash and equality predicate (stateful) objects

Consider a usecase with stateful comparators and hashers where they both share the same state (IOW they are the same object providing two functions - one for hashing and one for comparison) - would you consider adding this possibility?
For example by adding an empty 'flag' comparator type SameAsHasher (or something to that effect)?

(to avoid the duplication or 'smaller' duplication where the hasher and comparator store a reference to an external object as well as the associated more complex codegen)