pcase.el (\`): Try and handle large patterns better

pcase.el (\`): Try and handle large patterns better

Large backquote patterns tend to lead to very large and deeply
nested expansions, but they also tend to contain a lot of
"constant" subpatterns that can be compiled to quote patterns.
This patch does just that.  See discussion at
https://lists.gnu.org/archive/html/emacs-devel/2024-05/msg01140.html

* lisp/emacs-lisp/pcase.el (pcase--split-pred):
Improve the handling of pred-vs-quote so it also works with
quoted objects like cons cells, vectors, and strings.
Simplify the `pcase--mutually-exclusive-p` branch accordingly.
(pcase--expand-\`): New function, extracted from the \` pcase macro.
Make it recurse internally, and optimize backquote patterns to `quote`
patterns where possible.
(\`): Use it.

* test/lisp/emacs-lisp/pcase-tests.el (pcase-tests-vectors): Add tests
that were broken by a more naïve version of the optimization.
(pcase-tests-quote-optimization): New test.