[Feature Request] add optional #:key-comparator to for/hash and potentially #:type

[sorawee]

#:key-comparator accepts a value, which can be either 'equal, 'eq, 'eqv, or 'equal-always, defaulting to 'equal when the clause is not provided.

#:type accepts a value, which can be either 'immutable, 'mutable, 'weak, or 'ephemeron, defaulting to 'immutable when the clause is not provided.

Can #:type 'weak ever be practically useful though? I guess users need to stash the key values somewhere to retain the reference.