feature: Add f16 feature

[FL33TW00D]

It would be very handy to be able to use f16 with Arbitrary.

We are integrating f16 into wgpu which makes extensive use of this crate! gfx-rs/wgpu#5701

[fitzgen]

I think this should go the other way: the half crate should add an optional arbitrary feature (the same way you wouldn't expect serde to add an optional half dependency, but half to add an optional serde dependency instead).