Use predication intrinsics (i.e. CMOV) for `ConditionallySelectable` about subtle HOT 2 OPEN

I wrote up this idea a bit more in #107 (comment).

The CMOV family on x86/x86_64 can be used to implement ConditionallySelectable::conditional_assign, and the CSEL family can be used on aarch64.

Once you have this defined, ConstantTimeEq can be implemented in terms of it, by XORing the inputs and using e.g. cmovnz

from subtle.

I second this, as the implementor of CmovEq and just generally helping the crate out, I do have a working custom-time module based on cmov alone - the dudect-bencher for Rust outputs extremely low T values, which idicates that it does indeed work and runs in constant-time, across a variety of tests.

Getting assured, guaranteed branch-less code into a crate such as this would be a game-changer, IMO - I don't mind doing any of the heavy lifting in my free time either.

from subtle.