electric-layout-mode kicks in before electric-pair-mode

This aims to solve problems with indentation.  Previously in, say, a
js-mode buffer with electric-layout-rules set to

   (?\{ before after)
   (?\} before)

would produce an intended:

   function ()
   {
     <indented point>
   }

The initial state

  function () {

Would go immediately to the following by e-p-m

  function () {}

Only then would e-l-m be applied to } first, and then again to {.
This makes lines indent in the wrong order, which can be a problem in
some modes.

The way we fix this is by reversing the order of e-p-m and e-l-m in
the post-self-insert-hook (and also fixing a number of details that
this uncovered).  In the end this changes the sequence from

  function () {

By way of e-l-m becomes:

  function () <newline>
  {
  <newline>

The e-p-m inserts the pair

  function () <newline>
  {
  <newline>}

And then e-l-m kicks in for the pair again, yielding the desired result

  function () <newline>
  {
  <indented point>
  }

* lisp/elec-pair.el (electric-pair--insert): Bind
electric-layout-no-duplicate-newlines.
(electric-pair-inhibit-if-helps-balance)
(electric-pair-skip-if-helps-balance): Use insert-before-markers,
playing nice with save-excurion.
(electric-pair-post-self-insert-function): Go to correct position
before checking electric-pair-inhibit-predicate and
electric-pair-skip-self predicate.
(electric-pair-post-self-insert-function): Increase priority to
50.

* lisp/electric.el (electric-indent-post-self-insert-function):
Delete trailing space in reindented line only if line was
really reindented.  Rewrite comment.
(electric-layout-allow-duplicate-newlines): New variable.
(electric-layout-post-self-insert-function-1): Rewrite comments.
Honours electric-layout-allow-duplicate-newlines.  Don't reindent
previous line because racecar.

* test/lisp/electric-tests.el: New test.
(plainer-c-mode): Move up.
(electric-modes-int-main-allman-style)
(electric-layout-int-main-kernel-style): Simplify
electric-layout-rules.
(electric-layout-for-c-style-du-jour): New helper.
(electric-layout-plainer-c-mode-use-c-style): New test.

lisp/elec-pair.el

@@ -227,7 +227,8 @@ inside a comment or string."
 (defun electric-pair--insert (char)
   (let ((last-command-event char)
 	(blink-matching-paren nil)
-	(electric-pair-mode nil))
+	(electric-pair-mode nil)
+        (electric-layout-allow-duplicate-newlines t))
     (self-insert-command 1)))
 
 (cl-defmacro electric-pair--with-uncached-syntax ((table &optional start) &rest body)
@@ -426,11 +427,10 @@ happened."
                            (eq (cdr outermost) pair)))))
                  ((eq syntax ?\")
                   (electric-pair--unbalanced-strings-p char))))
-       (insert-char char)))))
+       (insert-before-markers char)))))
 
 (defun electric-pair-skip-if-helps-balance (char)
   "Return non-nil if skipping CHAR would benefit parentheses' balance.
-
 Works by first removing the character from the buffer, then doing
 some list calculations, finally restoring the situation as if nothing
 happened."
@@ -452,7 +452,7 @@ happened."
                             (not (eq (cdr outermost) pair)))))))
                  ((eq syntax ?\")
                   (electric-pair--inside-string-p char))))
-       (insert-char char)))))
+       (insert-before-markers char)))))
 
 (defun electric-pair-default-skip-self (char)
   (if electric-pair-preserve-balance
@@ -498,7 +498,9 @@ happened."
         ((and (memq syntax '(?\) ?\" ?\$))
               (and (or unconditional
                        (if (functionp electric-pair-skip-self)
-                           (funcall electric-pair-skip-self last-command-event)
+                           (save-excursion
+                             (goto-char pos)
+                             (funcall electric-pair-skip-self last-command-event))
                          electric-pair-skip-self))
                    (save-excursion
                      (when (and (not (and unconditional
@@ -525,8 +527,10 @@ happened."
         ((and (memq syntax '(?\( ?\" ?\$))
               (not overwrite-mode)
               (or unconditional
-                  (not (funcall electric-pair-inhibit-predicate
-                                last-command-event))))
+                  (not (save-excursion
+                         (goto-char pos)
+                         (funcall electric-pair-inhibit-predicate
+                                  last-command-event)))))
          (save-excursion (electric-pair--insert pair)))))
       (_
        (when (and (if (functionp electric-pair-open-newline-between-pairs)
@@ -540,7 +544,7 @@ happened."
                       (matching-paren (char-after))))
          (save-excursion (newline 1 t)))))))
 
-(put 'electric-pair-post-self-insert-function   'priority  20)
+(put 'electric-pair-post-self-insert-function   'priority  50)
 
 (defun electric-pair-will-use-region ()
   (and (use-region-p)