Added skip_exactly.
Added indexed.
Added Clone and Show to iterators where possible.
Added missing size_hint to TakeExactlyItems.
Added RandomAccessIterator impls where amenable.
Added chunks and chunks_exact.
... and immediately removed them.

The problem is that I can't see an efficient way of implementing them
*without* allocating for every chunk.  You can't return a slice of an
internal buffer, insofar as I can see.

I think this is the same thing stopping StreamingIterator from being
merged into Iterator.
Modified sequence! and recurrence! to *not* wrap around.
Added accumulate.

It was *definitely never* called scanl for two minutes.