draft: Optimize Option<CompactStr> to be the same size as CompactStr

[ParkMyCar]

This is a proof of concept to show that it's possible in stable Rust to get std::mem::size_of<CompactStr> == std::mem::size_of::<Option<CompactStr>>().

Unfortunately as-is the performance of CompactStr is degraded. We would need to fix this before merging.

Fixes #19

[zlstringham]

How attached are you to the Packed representation? I think you can accomplish everything you want without the ReprWithNiche or the std::mem::transmute, etc., if you can drop Packed.

Then I think you can extract the discriminant out of the union, and use a NonMaxU8 for it; e.g.

pub struct DiscriminantMask {
    val: NonMaxU8,
}

pub struct Repr {
    mask: DiscriminantMask,
    inner: ReprInner,
}

pub union ReprInner {
    heap: ManuallyDrop<HeapString>,
    inline: InlineString,
}

I think there will be some amount of refactoring (e.g. working with the size value of Inline outside of the union) but I don't think that's major.

[ParkMyCar]

I'm pretty attached to the Packed representation. Specifically because that allows you to inline 12 ASCII character strings on 32-bit machines, and 12 characters is the most common length for filenames because of the 8.3 notation from the early days of FAT, e.g. DSC_1234.jpg.

That being said, maybe we can use the layout you proposed and only transmute for the Packed repr. In the common case this removes the transmute and thus the extra copy, at the cost of code complexity 🤔

[ParkMyCar]

Completed in #105