New seq-contains-p predicate (Bug#34852)

New seq-contains-p predicate (Bug#34852)

* lisp/emacs-lisp/seq.el (seq-contains-p): New predicate function.  It
is a replacement for seq-contains which cannot be used as a predicate
when a sequence contains nil values as it returns the element found.
(seq-contains): Make obsolete.

* test/lisp/emacs-lisp/seq-tests.el (test-seq-contains-p):
(test-seq-intersection-with-nil, test-seq-set-equal-p-with-nil,
test-difference-with-nil): Add regression tests.

* doc/lispref/sequences.texi (Sequence Functions): Document
seq-contains-p.