Memoizes a function, using a custom function to hash the arguments.
Private for now until:
- There are other places in the codebase that could benefit from this
(e.g. LSP), but might require other changes to accommodate.
- Invalidation of the cache needs to be controllable. Using weak tables
is an acceptable invalidation policy, but it shouldn't be the only
one.
- I don't think the story around `hash_fn` is completely thought out. We
may be able to have a good default hash_fn by hashing each argument,
so basically a better 'concat'.
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