Support wasm `simd128`

**Is your feature request related to a problem? Please describe.**
If the `simd128` target feature is enabled and the arch is wasm, the SimdBlock size should be 16 bytes (128 bits).

Tracking issue: #17

Describe the solution you'd like
Change in the src/alignment/simd.rs file and a test asserting the size when compiled on wasm with `simd128` enabled.