Update ndarray to Rust 2018 edition

[jturner314]

Most of the changes are adding a crate prefix to use statements and changing try! to ?. I also renamed libnum to num_traits because cargo fix was wanting to add a crate prefix, e.g. use crate::libnum::Whatever, and I didn't see any reason for the num-traits crate to be renamed to libnum anyway.

This PR changes only the ndarray crate; it doesn't change the ndarray-rand crate.

Edit: Also note that I've just run cargo fix --edition and made some manual improvements. I haven't run cargo fix --edition-idioms, because according to the docs, --edition-idioms isn't ready yet.

[bluss]

I'm very happy with the new path rules so far. Rust 2018 ftw

[hellow554]

Is there a 0.13 planned for the edition bumb? Or do you still want to push it as 0.12.1?

[bluss]

Several breaking changes merged and coming, so it's a bump