Rework elisp-shorthands to only allow only prefix substitution

Rework elisp-shorthands to only allow only prefix substitution

This simplification in requirements makes for more complex C code but
that code is much less wasteful in Lisp strings than the previous
implementation.

* src/lread.c (read1): Rework.
(Fintern): Rework.
(Fintern_soft): Rework.
(Funintern): Rework.
(oblookup_considering_shorthand): Rewrite.

* test/lisp/progmodes/elisp-mode-tests.el (elisp-shorthand-read-buffer)
(elisp-shorthand-read-from-string): Use new format of
shorthand-longhand-.

* test/lisp/progmodes/elisp-resources/simple-shorthand-test.el (f-test)
(f-test2, f-test3): Use new form of elisp-shorthands.