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.
writeFile was deprecated in favor of writeFile2 in f645022d16361865e24582d28f1e62312fbc73bb. This commit renames writeFile2 to writeFile and makes writeFile2 a compile error.
