**Description:**
`pmutil` born to die. Now almost all useful features are merged into `syn` and `quote`, so it's now just a compile-time bloat. (Although I didn't send PRs to them, sorry)
- Rename `AstKind` to `AstParentKind`.
- Add `kind()` to `AstNodeRef`.
- Reduce generic arguments of `AstNodePath`.
- Remove `Opt*` from `AstParentKind` and `AstNodeRef`.
- Add field enum for structs.
- Add field enum for enums.
- Change `Vec*` to a child kind.
- Add field for the child field to `AstParentKind`.
- Add field for the child field to `AstNodeRef`.
