feat: split Chain into a struct and ChainKind enum

[DaniPopes]

Instead of checking whether Self::Id(_) can be converted into a NamedChain everywhere, we can enforce this at construction by making Chain a struct with a hidden inner ChainKind enum.

By making the fields private, we also make room for potential optimizations in the future, such as reducing the size of Chain from 16 to 8 bytes.

[mattsse]

I like this

pending @Evalir

[mattsse]

can we move this below the struct def?

[DaniPopes]

I prefer trait impls blocks before main impl, that's how I did it in all of core

[mattsse]

I'm used to above trait impls -.-

but I don't mind

[Evalir]

Super nice. I like implemeting traits first too, so agree

By making the fields private, we also make room for potential optimizations in the future, such as reducing the size of Chain from 16 to 8 bytes.

ahhh we're optimizingg