Blender Git Loki

BLI: support looking up a key stored in Map or VectorSet

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.