reading npz files takes soooooo long

[ExpHP]

Reading an .npz file is done through a child Python process, and the interprocess communication for this appears to be incredibly time-consuming.

Writing files is fine; nearly instant. Not sure why it's so asymmetric.

I've long wanted to substitute this out for rust code that reads npz files, but unfortunately my PRs to npy-rs have fallen into limbo. (and those are just the beginning of the work necessary to read and write npz files) Some day when I have time I'm gonna finish and publish my fork of that crate.

[ExpHP]

I just published https://github.com/ExpHP/npyz version 0.6 which ought to have everything we need to read the files natively in rust