Rust float-point in constant context

Floating-point code is from compiler_builtins = "0.1.87" and libm = "0.2.6", and has been rewritten for use in a constant context.

Some fuzzing of operations is performed to ensure correctness of the port, but please open an issue if there is any inconsistent behavior.

Features:

• no_std
• const_trait_impl
• const_mut_refs

On stable:

const fn const_f32_add(a: f32, b: f32) -> f32 {
    SoftF32(a).add(SoftF32(b)).to_f32()
}

On nightly with const_trait_impl usage:

const fn const_f32_add(a: f32, b: f32) -> f32 {
    (SoftF32(a) + SoftF32(b)).to_f32()
}

On nightly with const_mut_refs usage:

const fn const_f32_add(a: f32, b: f32) -> f32 {
    let mut x = SoftF32(a);
    x += SoftF32(b);
    x.to_f32()
}