Fix bevy_math when no alloc is available

[NiseVoid]

Objective

bevy_math does not work when no alloc is available despite the existance of the alloc feature which should gate all logic that requires it. Support for no std + no alloc can be useful for some embedded usecases, for example using bevy_math types on the GPU using Rust GPU.

Solution

Swap smallvec out with arrayvec, and fix two minor uses of alloc::slice::sort_by (not available on core because they allocate Vecs internally)

Testing

• Unit tests still pass
• I was able to compile a shader with Rust GPU while depending on bevy_math