* lisp/emacs-lisp/pcase.el: Use pcase-defmacro to handle backquote.

* lisp/emacs-lisp/pcase.el: Use pcase-defmacro to handle backquote.
(pcase--upat): Remove.
(pcase--macroexpand): Don't hardcode handling of `.
(pcase--split-consp, pcase--split-vector): Remove.
(pcase--split-equal): Disregard ` since it's expanded away.
(pcase--split-member): Optimize for quote rather than for `.
(pcase--split-pred): Optimize for quote rather than for `.
(pcase--u1): Remove handling of ` (and of `or' and `and').
Quote non-selfquoting values when passing them to `eq'.
Drop `app's let-binding if the variable is not used.
(pcase--q1): Remove.
(`): Define as a pattern macro.