Feature request: Add `get_or_compute`/`compute_if_absent` function

Inspired by computeIfAbsent from Java;

• Search for a value by key.
• If no value is found, compute a value and insert it.
• Return a reference to the value.

For Sets, the expected behavior when they key is not found is to insert into a separate entry, rather than populating the entry associated with the key.

This would provide an easy and efficient path for people looking for this behavior, and avoid misuse of get_or_insert_with, which appears to do the same thing, but doesn't, see #399 and the discussion in #396.

I propose the name get_or_compute rather than following Java, since it makes it harder to accidentally use get_or_insert_with because autocomplete will suggest both.

Start by reviewing the existing get_or_insert_with behavior and the discussions in #399 and #396; the issue does not name files or tests. Clarify the API and map/set semantics, then add coverage showing lookup, computation, insertion, and returned references behave as specified.