Option<Float> Niche optimization

[novacrazy]

Implements #283 as both a raw Option<f32> -> ArchivedOptionF32 low-level wrapper and as a with::Niche/ArchiveWith implementation using noisy_float::NoisyFloat for safety.

noisy_float is an optional dependency, and a simple FloatChecker trait has been included with option_float for non-noisy_float usage, but this is automatically implemented for all noisy_float::FloatChecker types if enabled.

Some of the generated docs are bit off with the lowercase type names, so I may fix those later. Fixed.

Future work may be to include all F: Float types instead of only f32 and f64, but that would require work with rend or others to implement Float or whatnot.

Starting this as a draft so I can review it again in the morning and fix anything I missed.

Closes #283

[djkoloski]

Hmm, I see the problem here; we don't really have a niche-able float type to bridge from. I really like your approach of adding a checker type parameter that decides whether a value is valid or not, I think that we might even want to generalize it. I could see something like this:

union NichedOption<T, D> {
    some: T,
    none: (),
}

trait Decider<T> {
    /// # Safety
    ///
    /// `ptr` must be non-null, properly-aligned, and be valid for reads. `ptr` does not have to point to a valid `T`.
    unsafe fn is_some(ptr: *const T) -> bool;
}

Which should allow us to generalize the Niche wrapper:

struct Niche<D>(PhantomData<D>);

struct Zero;

impl Decider<NonZeroU32> for Zero { .. }

struct NaN;

impl Decider<f32> for NaN { .. }

// etc.

It would be really nice to unify all of this niching code. What do you think? I understand if you'd rather not pivot like this, since it's a pretty significant change in scope.