Speed up unintern, and fix symbol shorthand edge case (bug#79035)

Speed up unintern, and fix symbol shorthand edge case (bug#79035)

Don't do a full lookup if the argument is a symbol, and only compute the
hash index once.  Fix a bug that occurred when there is another symbol
whose shorthand is equal to the true name of the symbol being removed.

* src/lread.c (Funintern): Rewrite for speed and correctness.
(oblookup_last_bucket_number, oblookup): Remove now unused variable.
* test/src/lread-tests.el (lread-unintern): New test.

(cherry picked from commit f4a9673f615aa8d1fad499784fdcd11ac0ec4042)