Implement pretty display of sub- and superscripts.

Implement pretty display of sub- and superscripts.

* doc/org.texi (Subscripts and superscripts): Document that `C-c C-x \'
will also format sub and superscripts.
* doc/orgcard.tex: Document that `C-c C-x \'
will also format sub and superscripts.
* lisp/org.el (org-use-sub-superscripts):
(org-pretty-entities-include-sub-superscripts): Move here from
org-exp.el.
(org-set-regexps-and-options): Parse subscript option and set
`org-use-sub-superscripts' as a local variable.
(org-match-sexp-depth):
(org-create-multibrace-regexp):
(org-match-substring-regexp):
(org-match-substring-with-braces-regexp): Moved here from org-exp.el
(org-set-font-lock-defaults): Call `org-raise-scripts'.
(org-remove-font-lock-display-properties): New function.
(org-unfontify-region): Call
`org-remove-font-lock-display-properties'.
(org-script-display): New constant.
(org-raise-scripts): New function.

When turning on entity display with `C-c C-x \', sub- and superscripts
will also be displayed in a smaller font, and raised/lowered.