1.0.12.29: optimize POSITION & FIND families for strings

1.0.12.29: optimize POSITION & FIND families for strings

  * If the string is simple, and the element type is know the old code
    does well already. Add a dispatch for (SIMPLE-ARRAY CHARACTER (*))
    to the out-of-line versions on unicode builds.

  * To keep code size down to something reasonable, remove the
    dispatch for SIMPLE-VECTOR: presumably POSITION & FIND are much
    more common on strings of uncertain type, then random vectors.