Add `save-mark-and-excursion', which has the old `save-excursion' behavior

Add `save-mark-and-excursion', which has the old `save-excursion' behavior

* doc/lispref/positions.texi (Excursions): Document
`save-mark-and-excursion'.

* lisp/font-lock.el (font-lock-fontify-block): Use
`save-mark-and-excursion' instead of `save-excursion', restoring
Emacs 24 behavior.

* lisp/simple.el (save-mark-and-excursion--save)
(save-mark-and-excursion--restore): New functions.
(save-mark-and-excursion): New user macro.

* src/editfns.c (Fsave_excursion): Mention
`save-mark-and-excursion' in `save-excursion' documentation.