Refactor INTERN and FIND-SYMBOL.

Avoid or delay copying to a simple-string when given a string
with fill-pointer as the input.

Fix a bug that one of the string.pure tests succeeded for the wrong
reason: (INTERN (MAKE-ARRAY 0 :ELEMENT-TYPE NIL)) reached an unhandled
case of STRING-DISPATCH, but a previous call to (INTERN "") hid it.
Threadsafe most-recently-used package optimization in FIND-SYMBOL*

This was unquestionably a win despite comments casting doubt.