Roll our own matrix code & separate math into new crate

[FreezyLemon]

Closes #24.

I hope it's ok to have these (multiple) changes in one PR. The new crate is minimal but should include everything that's needed for distribution, I think.

Clippy lints for yuvxyb_math are copied from yuvxyb/src/lib.rs. CI for the subcrate is also very simple because the build & msrv are transitively confirmed via yuvxyb (-math is a non-optional dependency).

I'll run some benches soon(TM), but this PR is otherwise done in my eyes.

[shssoichiro]

I like the #[must_use] additions. tbh I wish the compiler automatically added these for functions with no mutable arguments and a returned value. or maybe there's a clippy lint to enforce adding it.

[FreezyLemon]

I like the #[must_use] additions. tbh I wish the compiler automatically added these for functions with no mutable arguments and a returned value. or maybe there's a clippy lint to enforce adding it.

There actually was a clippy lint for it, funnily enough. I think it's because I moved the structs to a public API (instead of pub(crate) etc.).

One useful thing is to mark a type as must_use. That way, any public API returning it will be must_use as well.