math.combinatorics: much faster permutations, cleanup combinations code.
math.combinatorics: cleanup stack effects to be more descriptive.
math.combinatorics: minor fix to reduce-combinations stack effect.
io.files.info: Add file? and symlink? words.
math.combinatorics: fix map>assoc-combinations (shouldn't have been locals).
io.files.info: Longer names for file-info predicates.