A simpler variant of rust-lang/portable-simd#206.
* Comparisons are moved to `SimdPartialEq`, `SimdPartialOrd`, and `SimdOrd`. The function names are prefixed with `simd_` to disambiguate from the regular `PartialEq` etc functions. With the functions on traits instead of `Simd` directly, shadowing the function names doesn't work very well.
* Floating point `Ord`-like functions are put into a `SimdFloat` trait. The intention is that eventually (some time after this PR) all floating point functions will be moved from `Simd` to `SimdFloat`, and the same goes for future `SimdInt`/`SimdUint` traits.
1ec010db2a