Generalized rows/columns producers

[bluss]

.inner_iter() as it is today, is a generalized rows iterator. It produces all rows, even for more-dimensional arrays. What “rows” means is then the last axis, Axis(1) in a 2D array and Axis(n - 1) in an nd array.

The method names rows(), columns() are taken (for row counts and column counts). We can't use row_iter as the name, because it will return a rows producer and iterable, not iterator.

There is also the choice of what generalized columns should mean (Always the outermost axis, or always the next to last axis?)

(Break because inner_iter will be renamed and generalized).

[bluss]

I decided it's natural that generalized columns = whatever's contiguous in generalized f-contiguous arrays. Which means Axis(0).