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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.