This code is based on rust_sse2.rs of the same distribution, and is
subject to further improvements. Some comments are left intact even if
their applicability is questioned.

SIMD implementation is gated by `wasm32-simd` feature, portable version
is used otherwise.

Performance measurements with a primitive benchmark with ~16Kb of data:

| M1 native     | 11,610 ns |
| M1 WASM SIMD  | 13,355 ns |
| M1 WASM       | 22,037 ns |
| x64 native    |  6,713 ns |
| x64 WASM SIMD | 11,985 ns |
| x64 WASM      | 25,978 ns |

wasmtime v12.0.1 was used on both platforms.

Closes BLAKE3-team#187.

Wasm SIMD API has certain function safe.

Certain functions' unsafety comes from v128 loads and store.  If
argument types guarantee that these loads and stores are safe, function
is declared safe, and internal unsafe blocks are commented.