Use core::num::NonZero where CT is not relevant

[dvdplm]

For variable time code, using core::num::NonZero<T> may be more performant than crypto-bigint::NonZero<T>. However, the stdlib version relies on the (perma-?)unstable trait ZeroablePrimitive, which means that we are restricted to Ts from Rust's standard numeric types. :/

See the FCP comment for some more details on what was recently stabilized and what was not.