Add “n-ary” zip

[bluss]

For given arities (2, 3, 4, ..) implement zip2, zip3, etc functions.

https://github.com/bluss/rust-ndarray/blob/master/examples/zip_many.rs

[bluss]

secrets from freude: they use a corresponding fold function as well.

[bluss]

• The Zip object should be splittable to enable fork parallelism (rayon)

[bluss]

Consider a generalization that fits very well into the current scheme:

Extend zip arguments to include everything that has a dimension and stride. For example an AxisIter is one-dimensional and has a uniform stride, supposedly.

A chunks iterator (of even chunks) is n-dimensional and has custom strides.