Optimise more inputs to `regexp-opt' (bug#36444)

Optimise more inputs to `regexp-opt' (bug#36444)

Use a more precise test to determine whether the input to `regexp-opt'
is safe to optimise when KEEP-ORDER is non-nil, permitting more inputs
to be optimised than before.  For example, ("good" "goal" "go") is now
accepted.

* lisp/emacs-lisp/regexp-opt.el (regexp-opt):
More precise test for whether the list is safe w.r.t. KEEP-ORDER.
(regexp-opt--contains-prefix): Remove.

* test/lisp/emacs-lisp/regexp-opt-tests.el: Use lexical-binding.
(regexp-opt-test--permutation, regexp-opt-test--factorial)
(regexp-opt-test--permutations, regexp-opt-test--match-all)
(regexp-opt-test--check-perm, regexp-opt-test--explain-perm)
(regexp-opt-keep-order): Test KEEP-ORDER.