Make the entire crate compatible with the array reference type

[akern40]

This PR is large, but is repetitive and contains all of the code necessary to make ndarray-stats compatible with ndarray 0.17. It does the following:

• Bump ndarray to 0.17.1
• Bump ndarray-rand to 0.16.0
• Implement the extension traits on ArrayRef instead of ArrayBase
• Remove the "storage" generics on the extension traits
• Update documentation to replace references to ArrayBase

[nilgoyette]

Interesting to see that the new ndarray makes most generic functions declaration less verbose. I mean, you already knew, but it's great to see it :)

[akern40]

Oh believe me it was satisfying! Just finished this same process for -linalg, which was the same but more