Sometimes it is useful to find the key that compares equal
to a known key. Typically that happens when the key itself
has additional data attached that is not part of its hash.
Note that the returned key reference/pointer is const, because
the caller must not change the key in a way that changes its
hash or how it compares to other keys.
This adds two new methods:
* `clear` just removes all keys from the vector set.
* `index_of_or_add` returns the index of a key and adds it if has not
been added before.
