add a flag to enable subst01 in selectznz

[JasonGross]

As a start, looking at the Selectznz in curve25519.go, shortening the lifetimes of the variables will help.

I'll aim to add a flag that allows this shortly. If it's helpful, I can also have the cmovznz calls inlined, though I expect that'll take me a little bit more work

Originally posted by @JasonGross in golang/go#40171 (comment)