consider adding `hashof` to immutables

This came out of @Chriscbr 's recent cachebale function proposed example. Since Wing is heavy on immutables, compiler internally can efficiently generate unique hashes for all sorts of immutable objects. That way users can access this hash through something like hashof to do cache-keying. We need to implement this anyway internally for the compiler in order to be able to efficiently do equality checks between immutables, so might as well expose it to the user through a global hashof in the MVP spec to stay consistent with other xxxxof(...) globals. This would be equivalent to Java's Object.hashCode or similar mechanic in Python.

Review the compiler work needed for immutable hashing and equality, along with the MVP specification, using the cacheable-function example as context. Define the user-facing hashof behavior and verify that it supports cache-keying while remaining consistent with the compiler’s internal equality requirements.