* lisp/emacs-lisp/pcase.el: Bind all the vars in `or` patterns

* lisp/emacs-lisp/pcase.el: Bind all the vars in `or` patterns

Improve the handling of `or` patterns where not all sub-patterns bind the
same set of variables.  This used to be "unsupported" and behaved in
somewhat unpredictable ways.

(pcase--expand): Rewrite.
(pcase-codegen): Delete.

* doc/lispref/control.texi (pcase Macro): Adjust accordingly.
Also remove the warning about "at least two" sub patterns.
These work fine, AFAICT, and if not we should fix it.

* test/lisp/emacs-lisp/pcase-tests.el (pcase-tests-or-vars): New test.