feature: Create a new heap variant to replace Arc<str>

[ParkMyCar]

This PR changes our HeapString implementation from using an Arc<str> to a custom defined ArcString type. This type is more analogous to Arc<[u8]>, specifically it allocates a buffer which may be larger than the "string" it contains. This gets us closer to being a true drop-in replacement for String and allows us to write APIs for mutating a CompactStr

[NobodyXu]

I think it’s totally unnecessary to reinvent Arc here.

We can use slice-dst to create an unsized Arc.

Or, we can use triomphe which provides similar functionality but easier to use, and I had my PR relating to MaybeUninit usage merged in it.

[ParkMyCar]

Thanks for the feedback @NobodyXu!

I did look at tripomphe and I actually saw your recent PR for adding an API for MaybeUninit<T>, but unfortunately it doesn't seem to have been released yet? Otherwise I played around with trimophe::ThinArc a bit but it doesn't provide an API to modify the underlying buffer, which is one of the goals of this change, so we can get closer to being a truly drop-in replacement for String.

[NobodyXu]

an API to modify the underlying buffer

Is that for the copy-on-write usage?

Well then, I think this does require creation of another unique Arc type, though I think it might be better to implement this in another crate in a generic way.

But then, that can be postponed until this feature is completed in compact_str.

This is quite a novel way to implement CompactStr to save memory while permit modification.

Thanks for taking your time to answer my question!