A general-purpose function-builder. EXPR should be of the
form (OPERATOR . ARGUMENTS). Will return a function equivalent
to (lambda (arg0 arg1 ... arg1) (mapcar #'fn arguments)).
e.g. (fn (and integerp oddp)) => (lambda (x) (and (integerp x) (oddp x))).
U:COMPOSE is treated specially, so that (fn (u:compose integerp oddp)) =>
(lambda (x) (integerp (oddp x))). (This is the same as (fn (integerp (oddp))))