Adjust previous electric.el and elec-pair.el change

Adjust previous electric.el and elec-pair.el change

This fixes a serious bug introduced previously
electric-pair-inhibit-if-helps-balance and
electric-pair-skip-if-helps-balance, whereby "innocent" markers were
being pushed by those function's new save-change-and-restore
semantics.  The fix can probably still be improved.

It also adds comments to parts of the code, where deemed necessary.

* lisp/elec-pair.el (electric-pair--insert): Add comment.
(electric-pair--save-literal-point-excursion): New helper macro.
(electric-pair-inhibit-if-helps-balance)
(electric-pair-skip-if-helps-balance): Don't use
insert-before-markers since it may hurt other markers that have
nothing to do with the 'save-excursion'.
(electric-pair-post-self-insert-function): Use
electric-pair--save-literal-point-excursion.

* lisp/electric.el (electric-indent-post-self-insert-function):
Remove lexical variable.