[FR] Mutable access to stateful hashers and comparators

(like the implementation table_core class template provides)
I have an exotic use case where the state of the two subject objects references an external 'master' object which can potentially get moved around in memory/has an unstable reference (say as if stored in a std::vector) yet when I need to use the unordered container in question I know for certain that the 'master' object is 'pinned' - and would like to simply then update the reference stored in the hasher...

Yes - this opens up the can of worms of enabling the user the change the hasher in such a way that it potentially changes the hashes it returns - but this is already possible just not as easily (exactly by storing references in hashers) - so simply demand that hashes and comparators be [[ gnu::const ]] or else UB...?