Add an implementation of not to __m256i and possibly other types.

[raphaelcohn]

For feature parity with C implementations, we could consider a (const) implementation of not (!) for __m256i, and probably other types.

Justification, eg code examples in answer to https://stackoverflow.com/questions/42613821/is-not-missing-from-sse-avx?rq=1 use C's ~ operator.

[Amanieu]

Operators on vector types are a non-standard extension which we don't support.

[Lokathor]

We should support it long term, but that would be an RFC debate of its own.

[raphaelcohn]

@Amanieu @Lokathor thank you. I won't be raising a RFC for this.