ctutils: split CtSelect and CtAssign? #1352

CtSelect has both methods ct_select and ct_assign, where the latter is provided (there's also a ct_swap).

CtSelect also as a Sized bound, so it can't be impl'd for e.g. slices. But a hypothetical CtAssign trait (which could also have a ct_swap method) has no need for a Sized bound.

It annoyingly means that there are two traits to impl instead of one, but while we could add a blanket impl, there's not an efficient general implementation of either in terms of the other and it would be nice to permit efficient implementations of both. So I would suggest against a blanket impl.

One thing we could do though is CtSelect: CtAssign + Sized to ensure that any types with CtSelect impls also have CtAssign impls.

I was noticing this looking at the Cmov trait and how it doesn't have this problem.