Fix SEQUENCE:SEARCH, test seq. functions with user-defined sequences

Fix SEQUENCE:SEARCH, test seq. functions with user-defined sequences

Extending the tests in tests/seq.impure.lisp to user-defined sequences
revealed that the previous implementation produced incorrect results
for some inputs.

SEQUENCE:WITH-SEQUENCE-ITERATOR now accepts NIL in the list of
variables and generates ignored bindings for these elements.

The new implementation is also slightly faster (at least for the
inputs in tests/seq.impure.lisp).

fixes lp#1153312