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

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

Instead of referencing obarray directly, that function has to consider
a collection of completions which includes the shorthand versions of
some of the symbols.  That collection changes from buffer to buffer,
depending on the choice of elisp-shorthands.

To make this process efficient, and avoid needless recalculation of
the above collection, a new obarray-specific cache was invented.  The
Elisp variable obarray-cache is immediately nullified if something
touches the obarray.

* 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-mode-tests.el
(elisp-shorthand-completion-at-point): New test.

* test/lisp/progmodes/elisp-resources/simple-shorthand-test.el
(f-test-complete-me): New fixture.