1.0.44.1: more conservative CONCATENATE open-coding

1.0.44.1: more conservative CONCATENATE open-coding

  Don't fully open code for long strings, no matter what policy:
  constraint-propagation will go seriously nonlinear.

  Also optimize the open-coded form a bit. Use

    (SETF (AREF .STRING. (TRULY-THE INDEX (+ .POS. <constant>))) <char>)
    ...repeat...
    (INCF .POS. <constant>)

  instead of

    (SETF (AREF .STRING .POS.) <char>)
    (INCF .POS.)
    ...repeat...

  . Smaller code, easier on the constraint propagation, and a tiny
  bit faster too.