Simplify generics

[Noam2Stein]

The crate currently uses many type system workarounds to get around limitations in stable Rust. These workarounds do not hurt the usual Vec3<f32> use case, but they do add a large amount of complicated items to the public API, complicate generic usage and worsen compile times.

These are important improvements that should be made once they are possible on stable Rust:

• Make Alignment an enum and use const generics (requires adt_const_params).
• Remove T: Copy (requires unsafe impl Copy).
• Replace wide with std::simd (requires portable simd).
• Deduplicate functions with conflicting names (requires disjoint trait implementations).
• Remove Length<N>: SupportedLength (requires specialization).
• Add Backend default implementation and remove Scalar (requires specialization).
• Remove primitive traits (requires generic integers).