Fixpoint function with memoisation

@[implemented_by _private.Mathlib.Util.MemoFix.0.memoFixImpl, deprecated "use `MonadCacheT` and `checkCache`" (since := "2026-01-24")]

opaque memoFix {α β : Type} [Nonempty β] (f : (α → β) → α → β) : α → β

Takes the fixpoint of f with caching of values that have been seen before. Hashing makes use of a pointer hash.

This is useful for implementing tree traversal functions where subtrees may be referenced in multiple places.