Implementations for u8->u64 and i8->i64

[timwedde]

This is my first Rust-related PR, so apologies for any inadvertent mistakes.

I picked up dfdx yesterday but in my application I'm dealing with u8's and u16's a lot, and I figured it would be quite handy if we could properly interface them with Numpy instead of having to cast to f32/64 for the data exchange.

This PR adds support for the following dtypes:

• u8
• u16
• u32
• u64
• i8
• i16
• i32
• i64

I wasn't sure what the policy of this project is in regards to testing, so I added two basic save/load tests for u8 and i8 to prove the concept. Let me know if I need to add all of the other tests as well. That'd be quite a few permutations in this case, so I didn't think it worth it for the initial PR.

[ccaven]

For future reference, make sure to run cargo fmt so the automated testing doesn't give you a "Some checks were not successful" :)

[timwedde]

Good tip, thank you, though in this case the failing check actually stems from clippy in a file I didn't even modify: src/nn/add_into.rs:5:1. Not sure why that is, but I guess it might be carry-over from some other commits on master before I forked and made this PR.
cargo fmt does nothing in my case, so I assume everything is in order, at least formatting-wise.

[coreylowman]

@timwedde looks good to me, can you resolve conflicts and then I can force merge if pipelines don't pass

[timwedde]

Alright, rebased onto main and checks all seem good.

[coreylowman]

Thanks! 🎉