This adds support for nesting `Annotated`/`NewType` wrappers in any
combination. Previously you could only nest:
- `Annotated[Annotated[...], ...]` (the `Annotated` constructor automatically
  flattens this, no need for `msgspec` to handle this case.
- `Annotated[NewType(...), ...]`
It also refactors the type processing code in `_core.c` to be a little
cleaner (still not "clean", Python type annotations aren't the easiest
objects to consume).