use implementation's EMPTYP when available

Using the implementation's EMPTYP has the potential advantage of
supporting user-defined sequence types (as is the case with SBCL).

This commit tests for SBCL's SEQUENCE:EMPTYP and defines
ALEXANDRIA:EMPTYP in terms of it.

For other implementations and older SBCL versions, nothing changes.
use EMPTYP instead of e.g. PLUSP and LENGTH were appropriate

EMPTYP usually (except, maybe, for some user-defined sequence types) has
two advantages:

1. it has O(1) runtime while LENGTH can be O(n)

2. it can work on user-defined, infinite sequences (given implementation
   support and integration of ALEXANDRIA:EMPTYP with the
   implementation's mechanism as is the case for SBCL)
removed unused condition NO-EXTREMUM in sequences.lisp
cosmetic change in sequences.lisp
fix WITH-OPEN-FILE*
ignore temporary files in doc/ directory