Fix nalgebra schema

[max-heller]

Closes #181, using a slice-of-array flattening function borrowed from the const-unstable <[[T; N]]>::as_flattened()

[netlify]

✅ Deploy Preview for cute-starship-2d9c9b canceled.

Name	Link
🔨 Latest commit	5edcc9a
🔍 Latest deploy log	https://app.netlify.com/sites/cute-starship-2d9c9b/deploys/674320ec9b5e780008bfeea8

[jamesmunns]

Oh this is DEVIOUS. I was ready to come in and say that &[T] isn't the same as &[T; N], but I didn't even THINK of doing shenanigans like this only on the schema.

I want to poke around at this, but this is extremely clever, thank you!