f16 native support in Rust

[VariantXYZ]

Sorry in advance if I missed a tracking issue somewhere, the ones I've found seemed to briefly mention but not propose/drive native f16 specifically.

Specifically, I've seen:
#2629
#521
rust-lang/stdarch#344

I think there are plenty of reasons to support f16 as a native arithmetic type in Rust, but my primary use-case is ML inference for hardware that supports fp16 arithmetic (e.g. the Cortex-a55).

I've resorted to writing simple functions (multiply - add, dot products, etc...) to operate on _Float16 values in C and calling them because the half crate's conversion cost is really painful for anything low-latency/high frequency (audio processing). It is... far from efficient.

[VariantXYZ]

I should've read #2629 better! I'll just drop my comments in there and close this.