feat: split Chain into a struct and ChainKind enum (#5)

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.