lib: typesafe *_member()

lib: typesafe *_member()

This provides a "is this item on this list" check, which may or may not
be faster than using *_find() for the same purpose.  (If the container
has no faster way of doing it, it falls back to using *_find().)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>