Remove redundant LENGTH calls in NSUBSTITUTE[-IF[-NOT]]

Remove redundant LENGTH calls in NSUBSTITUTE[-IF[-NOT]]

NSUBSTITUTE, NSUBSTITUTE-IF and NSUBSTITUTE-IF used to contain

  (let ((length (length sequence))) ...

in the code path for lists. In the macroexpansion it became clear that
this recomputed and shadowed the LENGTH variables already established
by DEFINE-SEQUENCE-TRAVERSER.

A very brief test with list lengths between 5 and 500 suggests that
the speedup produced by removing the redundant computation can be
around 5 to 20 %.