NonZero* type constructors should be const fn

[dhardy]

E.g. NonZeroU32::new_unchecked is a const fn but NonZeroU32::new is not; this means that defining constant values is unsafe.

pub const X: NonZeroU32 = unsafe {
    NonZeroU32::new_unchecked(123)
};

[SimonSapin]

This depends on #49146.

[Mark-Simulacrum]

@rustbot modify labels: +A-const-fn.

Testing the new bot!

[m-ou-se]

This has been fixed in 1.47. NonZero*::new is const now.