Make NonZero constructors generic.

[reitermarkus]

This makes NonZero constructors generic, so that NonZero::new can be used without turbofish syntax.

Tracking issue: #120257

I cannot figure out how to make this work with const traits. Not sure if I'm using it wrong or whether there's a bug:

101 |         if n == T::ZERO {
    |            ^^^^^^^^^^^^ expected `host`, found `true`
    |
    = note: expected constant `host`
               found constant `true`

r? @dtolnay