Fix typo in comment in src/compiler/seqtran.lisp
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 %.
Lazy length computation in DEFINE-SEQUENCE-TRAVERSER

DEFINE-SEQUENCE-TRAVERSER used to unconditionally establish a binding
of LENGTH{,1,2} to the length of the processed sequence(s). This was
wasteful in the case of user-defined sequences whos implementations
could not access this information.

DEFINE-SEQUENCE-TRAVERSER now uses SYMBOL-MACROLET to provide LENGTH{,1,2}
symbols which evaluate to the respective sequence lengths, but the
sequence length is only computed when needed.

Uses of DEFINE-SEQUENCE-TRAVERSER now have to avoid forcing
LENGTH{,1,2} and thus END{,1,2} handling only appears in the list- and
vector-code-paths. The affected sequence functions are REDUCE,
DELETE[-IF[-NOT]], REMOVE[-IF[-NOT]], REMOVE-DUPLICATES,
DELETE-DUPLICATES, SUBSTITUTE[-IF[-NOT]], NSUBSTITUE[-IF[-NOT]],
COUNT[-IF[-NOT]], MISMATCH and SEARCH.
Declare types of END and TEST in N{LIST,VECTOR}-SUBSTITUTE-IF[-NOT]*

The types of END and TEST are known because the respective callers are
defined with DEFINE-SEQUENCE-TRAVERSER which checks/COERCEs
appropriately.