Add scientific notation Display impls

[SV-97]

Description

I just ran into a case where I wanted to format OrderedFloats in scientific notation using std::fmt::LowerExp and std::fmt::UpperExp. This adds delegate implementations for both of those traits.

Example:

use ordered_float::OrderedFloat;

fn main() {
    let n = OrderedFloat(10.2);
    println!("{:+.2e}", n); // prints "+1.02e1"
}

[mbrubeck]

Thanks! Published as version 3.2.0.