Path to stabilization

[lu-zero]

• have a verification crate like stdsimd-x86-verify
• cover a large batch of the altivec.h API (75% ?)
• write an RFC to document it.

[lu-zero]

This document https://github.com/OpenPOWERFoundation/Programming-Guides/blob/master/Intrinsics_Reference/ch_vec_reference.xml might be structured enough to be easily parsed.