OrderedFloat refactor

[Bromeon]

Following #863 (comment), this moves egui/util/float_ord.rs -> epaint/util/ordered_float.rs and implements Hash based on epaint::f32_hash().

Open questions:

1. Currently, f64::ord() simply truncates to f32, which may yield incorrect results for high precision floats. I see three solutions:

   • Make OrderedFloat generic on the float type. A bit more complex implementation, as we need to create traits to abstract over f32 and f64 for is_nan() etc.
   • Use always f32 (status quo) -- not recommended, can cause really nasty bugs
   • Use always f64 -- safe, but might incur minor performance penalty

2. Should this be part of the public API?
   I moved it to a different crate, so it can't be pub(crate) anymore. However, we can mark it #[doc(hidden)] if it should stay an implementation detail.

[emilk]

Let's keep it f32 only until a need for f64 actually arises. Then we can make it generic.

It's fine if it's part of the public API imho - it is hidden enough in mod util

[Bromeon]

There's currently one occurrence already:

/~https://github.com/emilk/egui/blob/master/egui/src/widgets/plot/items/mod.rs#L1309
The field Bar::value is of type f64, for consistency with the X/Y values.

let new_base_offset = if bar.value.is_sign_positive() {
    others
        .iter()
        .filter_map(|other_chart| other_chart.bars.get(index).map(|bar| bar.upper()))
        .max_by_key(|value| value.ord())
} else {
    others
        .iter()
        .filter_map(|other_chart| other_chart.bars.get(index).map(|bar| bar.lower()))
        .min_by_key(|value| value.ord())
};

[emilk]

Oh right! Then I suggest either OrderedFloat<T> or OrderedF32/OrderedF64 (probably simpler).

[Bromeon]

Went for OrderedFloat<T>, with T: Float as an internal bound. Float is a minimalistic generalization of f32 and f64 with just hash() and is_nan() for now; could be extended if other parts of the library need it.

Hash for OrderedFloat<f64> needed a hash implementation; I copied f32_hash() and adjusted it for f64. Let me know if I should implement this differently!

Also added some documentation and a usage example.