1.0.31.8: specialized out-of-line CONCATENATE for strings

1.0.31.8: specialized out-of-line CONCATENATE for strings

  https://bugs.launchpad.net/sbcl/+bug/417229

  CONCATENATE 'STRING was already decent when SPEED > SPACE thanks to
  open coding by the deftransform.

  Deal with low-speed policies by adding %CONCATENATE-TO-STRING and
  %CONCATENATE-TO-BASE-STRING and transforming to them when
  appropriate.