(pcase): New `_` syntax in pred/app functions

(pcase): New `_` syntax in pred/app functions

The current syntax for functions in `app` and `pred` patterns
allows a shorthand (F ARGS) where the object being matched is
added as an extra last argument.  This is nice for things like
(pred (< 5)) but sometimes the object needs to be at
another position.
Until now you had to use (pred (lambda (x) (memq x my-list)))
or (pred (pcase--flip memq my-list)) in those cases.
So, introduce a new shorthand where `_` can be used to indicate
where the object should be passed: (pred (memq _ my-list))

* lisp/emacs-lisp/pcase.el (pcase--split-pred): Document new syntax
for pred/app functions.
(pcase--funcall): Support new syntax.
(pcase--flip): Declare obsolete.
(pcase--u1, \`): Use `_` instead.
(pcase--split-pred): Adjust accordingly.

* doc/lispref/control.texi (pcase Macro): Document new syntax
for pred/app functions.

* lisp/progmodes/opascal.el (pcase-defmacro):
* lisp/emacs-lisp/seq.el (seq--make-pcase-bindings):
* lisp/emacs-lisp/eieio.el (eieio):
* lisp/emacs-lisp/cl-macs.el (cl-struct, cl-type):
Use _ instead of `pcase--flip`.
(cl--pcase-mutually-exclusive-p): Adjust accordingly.

* lisp/emacs-lisp/map.el (map--pcase-map-elt): Declare obsolete.
(map--make-pcase-bindings): Use `_` instead.