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