1.0.12.23: Optimize STRING-*-TRIM

1.0.12.23: Optimize STRING-*-TRIM

       * Add deftransforms for STRING(-LEFT|-RIGHT|)-TRIM of simple strings.
         As a sleazy benchmark trick, also optimize for constant character bags.
       * Rewrite the function versions of the string trimmers for more
         code reuse. New versions also no longer cons up a new string when
         no trimming needs to be done. (Allowed in the spec, as pointed out
         by Attila Lendvai)
       * Add tests.