Use rust-numpy for data IO

[clbarnes]

Currently goes from array-like -> numpy array -> python list -> rust vec -> rust type -> rust vec -> python list -> numpy array, which is a lot of copies and changing type information; plus vec-of-vec isn't an efficient data structure (although we don't index into the top vec so maybe it's not so bad).

Blockers

• rust-numpy officially supporting python 3.8 Update CI for python 3.8 PyO3/rust-numpy#123

Thoughts

Don't want to copy the array more than necessary, but we may need to clone it on the rust side to prevent releasing/mutating the original memory if we were to use Into.

[clbarnes]

Released in v0.19