Add #[inline] on fallback functions

[Shnatsel]

I've been inspecting the stack traces in a profiler and noticed that the fallback conversion functions are not inlined.

The #[inline] attribute in Rust is not transitive, and the fallbacks will not be inlined if they are not marked #[inline] explicitly.

I've verified that everything in the call stack up to this point is already marked #[inline].

[Shnatsel]

Cuts the execution of the following test case in half, without the nightly-only feature for using intrinsics:

use std::{time::Instant, hint::black_box};

const ITERS: u32 = 500_000;

fn main() {
    let data = gen_data();

    let start_time = Instant::now();
    for i in 0..ITERS {
        for f in &data {
            let input = *f;
            black_box(input);
            let result = half::f16::from_f32(input);
            black_box(result);
        }
    }
    println!("Took {} ms", start_time.elapsed().as_millis());
}


fn gen_data() -> Vec<f32> {
    (0u32..1000u32).map(|i| i as f32).collect()
}

[starkat99]

Felt they were too non-trivial to inline without profiling it first, just never got around to doing that, so thanks for doing it!