make the simd_runtime_generate macro explicitly unsafe

[verpeteren]

As discussed in this issue it would be better that the simd_xxx_generate macros are explictly unsafe.

This has an impact for consumers of this crate, so a (minor) version bump might be applicable.

[arduano]

What makes those generated functions explicitly unsafe? Isn't it only the fact that the cpu might not have those instructions?

If so, then isn't the runtime generate function completely safe, because it ensures at runtime that the instructions that it will use do exist?