COLLECT-IF*: Refactor.

1. Rename keyword: from -> start, below -> end.
2. Remove conditionals in loop.
KEYS: add unless keyword.

For consistency with LAZY-FOR/LFOR.
VALS: add unless keyword.

For consistency with LAZY-FOR/LFOR.
SUBSTITUTE*, SUBSTITUTE-IF* sequence: add dispatch for key=nil.

For CCL-1.9 (substitute 42 0 '(0) :key nil) causes an error.
(FYI, SBCL-1.2.0 is ok)
cf. CLHS function substitute, substitute-if, substitute-if-not.
Merge branch 'master' of https://github.com/tkych/cl-plus