Added _yes get_or_intern methods

[qm3ster]

Adds get_or_intern_yes, try_get_or_intern_yes, get_or_intern_static_yes, try_get_or_intern_static_yes methods that return an additional boolean that answers questions like: "Was this a new, never before seen string?", "Am I the first person to receive this key?", "Should I go compile a Regex or other string derivative and store it in a keyed-alike collection eagerly?", etc.

This allows doing additional things on the first time a string is seen, without hashing twice.

I updated the doctests to actually test the correctness of the bool returned, but didn't otherwise add tests.

I didn't touch the Interner trait because it's not sealed and adding the new methods to it would be a breaking change. (An honest default implementation would hash twice by calling Resolver::contains_key, not sure if that is something we'd want to publish.)

[qm3ster]

Oh, and obviously _yes is a stupid name and we should call it something else. And we need to confirm that this is the intuitive order in the returned tuple.

[Kixiron]

Tuple order is good, I think a name involving insert would be good since that‘s similar to the HashMap api