Make all Level::as_* methods available on all targets

[danderson]

Currently, Level::as_sse2, Level::as_neon and so forth are only implemented on the target where that feature could be available. For writing tests/benchmarks that want to run over all locally available levels, this means you have to combine the as_* checks with #[cfg] checks or you get a compile error.

Could we instead define all as_* methods on all targets, with the impls on mismatched targets unconditionally returning None ? That would also make it slightly easier to write dispatches to custom kernels for specific levels, because your general simd function can just as_neon() on all platforms instead of having to throw in target conditionals.

[Dr-Emann]

What would be the result in the as_neon case on an x86 machine?

1. Same as on arm, Option<Neon>? That would mean Neon would also need to exist on all targets.
2. Or maybe Option<Infailable>? Maybe odd to have a different return type, but has some nice benefits.

Or, maybe, just is_* functions that are always available, like is_fallback is?

[Shnatsel]

For that to be possible, the NEON level on x86 still needs to exist as a nameable type, but currently it's conditionally compiled to keep build times low.

You also need to be able to call methods on NEON without cfgs, otherwise your cfgs just move from the initial .as_neon() call to all uses of it, so the methods still need to exist - and we can't remove NEON from the return type either. So returning Option<Infailable> is not an option. And we would also need to keep all the methods around too, but the NEON intrinsics aren't available on x86, so we would have to could turn them into stubs that just call unreachable! on platforms where they're not available. That adds complexity and doesn't fully remove the compile time impact.

Given the solution space, I think the current situation is the least bad option. And you would need some conditional compilation to import intrinsics anyway. https://github.com/linebender/fearless_simd/blob/main/fearless_simd/examples/srgb.rs is not too bad as it is, since kernel! handles the cfgs for you.