Style edit in iwithout()
Added is_seqcont()
Added nth()
Added i?flatten()
Removed "not complete yet" mention next to docs

There are lots of docs now, so that does more harm than good.
Better docs for i?partition() and pairwise()

- moved non-descending test example to pairwise()
- added sliding window example to i?partition()
- changed partition() to ipartition() in all examples
Language style edit in i?reductions() docs