This commits adds the following distinct integer types to std.zig.Ast:
- OptionalTokenIndex
- TokenOffset
- OptionalTokenOffset
- Node.OptionalIndex
- Node.Offset
- Node.OptionalOffset

The `Node.Index` type has also been converted to a distinct type while
`TokenIndex` remains unchanged.

`Ast.Node.Data` has also been changed to a (untagged) union to provide
safety checks.