Internal type overhaul and general tidying up 🧹✨

This release sees a fairly significant overhaul of the internal types, which had become somewhat cumbersome and hard to work with. Situations in which the type system had to be forcibly subverted have been addressed, and several helper types have been extracted to simplify some of the more complex conditional type logic.

This release should not contain any breaking changes (all functional and type-level tests are passing), but with refactors this significant there's always a slim risk that something will break for someone, so I've made this a major release just to be safe.

If you do encounter any issues, please raise an issue and I'll take a look when I can.