Rename elisp-shorthands to read-symbol-shorthands

Rename elisp-shorthands to read-symbol-shorthands

The new name fits better in the family of variables that affect
the Lisp reader.

Suggested-by: Po Lu <luangruo@yahoo.com>
* doc/lispref/symbols.texi (Shorthands): Mention read-symbol-shorthands

* lisp/shorthands.el (hack-read-symbol-shorthands)
(hack-read-symbol-shorthands)
(shorthands-font-lock-shorthands): Use read-symbol-shorthands

* lisp/progmodes/elisp-mode.el (elisp--completion-local-symbols)
(elisp--completion-local-symbols)
(elisp-shorthands): Use read-symbol-shorthands

* src/lread.c:
(syms_of_lread): Define Vread_symbol_shorthands
(oblookup_considering_shorthand): Use Vread_symbol_shorthands.

* test/lisp/progmodes/elisp-mode-tests.el (elisp-shorthand-read-buffer):
(elisp-shorthand-read-from-string): Use read-symbol-shorthands

* test/lisp/progmodes/elisp-mode-resources/simple-shorthand-test.el
Use new symbol name read-symbol-shorthands.