Can anyone glance over this `FourCharCode` wrapper?

I ended up wrapping the Foundation type FourCharCode so I can manage its values better (in a Swift-y way). It's at:

https://github.com/CTMacUser/ClassicFourCharCode

and I made a "0.1.0" tag. I put in every applicable standard-library and Foundation protocol I thought of. I'm looking for mistakes or for missing functionality.

(I can't add MutableDataProtocol support because it adds both MutableCollection and RangeReplaceableCollection requirements, and the latter is impossible for a type whose purpose involves having exactly four elements.)

Making it a Collection is super weird to me. Like, yes, it has a four-byte representation; yes, there are even some properties that depend on that representation (“all lowercase belongs to Apple”, was it?) but those bytes aren’t useful individually. Oh well.