Merge rust-lang/portable-simd#243 from ./no-overflow-panic

Remove overflow panic from divrem and add basic docs to Simd<T, N>

This finishes normalizing Simd<T, N> to being approximately equivalent to Simd<Wrapping<T>, N> for all implemented operations I can think of. It also documents this fact, allowing this to close rust-lang/portable-simd#56.