Add array stacking methods

[bluss]

No description provided.

[bluss]

<[_]>::concat() can be an inspiration.

[vbarrielle]

As a data point, I have some stacking methods for sparse matrices in sprs, as free functions:
https://github.com/vbarrielle/sprs/blob/master/src/sparse/construct.rs#L231

If I follow you, taking inspiration from SliceConcatExt would mean having a trait with methods such as fn stack(self, axis: Axis), and possibly helper methods such as vstack.
Ie we would have stacking work as let stacked = [a, b, c].stack(Axis(0)). That's probably lighter than the free function option.

One point that's hard to address using slices though is type heterogeneity. With that design, it's not possible to stack an OwnedArray and an ArrayView together. Maybe some trait like IntoArrayView could help there, not sure.

[bluss]

Using a slice argument either way is an inspiration from concat() too, so a free function can work.

[vbarrielle]

FYI I've started working on this, I'll PR once I have a proof of concept.