Fix pcase rx pattern bugs

Fix pcase rx pattern bugs

Two unrelated bugs: A missing type check caused an error in rx
patterns for non-string match targets, and rx patterns did not work at
all in pcase-let or pcase-let*.

Second bug reported by Basil Contovounesios and Ag Ibragimov; fixes
proposed by Stefan Monnier.  Discussion and explanation in thread at
https://lists.gnu.org/archive/html/emacs-devel/2021-02/msg01924.html

* lisp/emacs-lisp/rx.el (rx): Add (pred stringp) to avoid type errors,
and replace the `pred` clause for the actual match with something that
works with pcase-let(*) without being optimised away.
* test/lisp/emacs-lisp/rx-tests.el (rx-pcase): Add test cases.