`NotNan::new` can be `const`

[bentheiii]

Hello. AFAICT, there's no reason for NotNan::new to not be const. This will allow users to safely unwrap float literals in a const context.

let my_default_non_nan_float = const {NotNan::new(0.5).unwrap()};

I'd be happy to make a PR if this is approved

[mbrubeck]

Two issues:

• f64::is_nan is const since Rust 1.83, but this crate currently supports older versions. I am willing to increase the required Rust version, but I might want to wait a bit before doing this.
• num_traits::FloatCore::is_nan is not const, so this can't be done in the existing generic impl. This might need to be a new method bounded on the PrimitiveFloat trait.