1.0.17.42: more string trimming fixes

1.0.17.42: more string trimming fixes

 * Return value in the no-op case for non-simple-strings must not be
   the underlying simple string object.

 * Correct return type (per spec) for STRING-TRIM &co is
   STRING-DESIGNATOR, but as long as we take care, we can make it
   STRING -- but the old SIMPLE-STRING is still wrong.

 * Instead of making WITH-STRING a full-blown Evil Macro, just use
   WITH-ARRAY-DATA at the call site.

 * Two more test-cases.

   patch by James Knight.