Consider shorthands in Elisp's elisp-completion-at-point

Consider shorthands in Elisp's elisp-completion-at-point

* lisp/progmodes/elisp-mode.el : new helper.
(elisp-completion-at-point): Use new helpers.
(elisp--completion-local-symbols)
(elisp--fboundp-considering-shorthands)
(elisp--bboundp-considering-shorthands): New helpers

* src/lread.c (intern_driver): Nullify Qobarray_cache.
(syms_of_lread): Add Qobarray_cache.

* test/lisp/progmodes/elisp-resources/magnars-string-user.el: New
file to play around with `magnars-string` library.