Move contents of builtins/* to stdlib/*. This simplifies finding stubs.

stdlib/2and3/marshal.pyi

@@ -0,0 +1,8 @@
+from typing import Any, IO
+
+version = ...  # type: int
+
+def dump(value: Any, file: IO[Any], version: int = ...) -> None: ...
+def load(file: IO[Any]) -> Any: ...
+def dumps(value: Any, version: int = ...) -> str: ...
+def loads(string: str) -> Any: ...