Consider removing math.rs from the public API

[FreezyLemon]

Most of these functions are not inherently tied to v_frame and are just util functions for rav1e. I'd argue that it makes sense to move them to rav1e instead.

The Fixed trait is also kind of unnecessary: Because of orphan rules, there is no way for downstream crates to implement the trait for the other integer types. This means that these functions could also just be simple (non-trait) functions operating on a usize parameter.

I'd suggest making the math.rs a util module and not exposing it publicly.