Add const fn constructors for NonZero<Uint> and NonZero<Limb>

[fjarri]

This will allow us to make a number of methods const fn (for example, various division methods) and introduce NonZero bounds where they were previously impossible (for example, inv_mod.rs methods where moduli are currently just &Self). Also it opens up a possibility to initialize a NonZero constant of a modulus/divisor known at compile-time, so that it doesn't have to be unwrapped every time on use.