Feature request: Add get_or_compute/compute_if_absent function

[yyny]

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.

[Amanieu]

I would prefer fixing the existing method rather than adding another which does almost the same thing. This should be discussed in rust-lang/rust#60896.