[racket] member et al.

Racket still offers several primitives, such as member, that are
primarily used as predicates, but don't actually return a Boolean.
Racket's cond handles any of these return values as true, but now that
Racket is gravitating towards contracts and typed modules, shouldn't
Racket's predicates actually return true or false?  If the existing
functions need to stick around for legacy purposes, wouldn't it be
beneficial to offer Boolean-guaranteed verisons as well, such as
member?, andmap?, ormap?, etc.