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

stdlib/2.7/cStringIO.pyi

@@ -0,0 +1,50 @@
+# Stubs for cStringIO (Python 2.7)
+# See https://docs.python.org/2/library/stringio.html
+
+from typing import overload, IO, List, Iterable, Iterator, Optional, Union
+from types import TracebackType
+
+# TODO the typing.IO[] generics should be split into input and output.
+
+class InputType(IO[str], Iterator[str]):
+    def getvalue(self) -> str: ...
+    def close(self) -> None: ...
+    @property
+    def closed(self) -> bool: ...
+    def flush(self) -> None: ...
+    def isatty(self) -> bool: ...
+    def read(self, size: int = ...) -> str: ...
+    def readline(self, size: int = ...) -> str: ...
+    def readlines(self, hint: int = ...) -> List[str]: ...
+    def seek(self, offset: int, whence: int = ...) -> None: ...
+    def tell(self) -> int: ...
+    def truncate(self, size: int = ...) -> Optional[int]: ...
+    def __iter__(self) -> 'InputType': ...
+    def next(self) -> str: ...
+    def reset(self) -> None: ...
+
+class OutputType(IO[str], Iterator[str]):
+    @property
+    def softspace(self) -> int: ...
+    def getvalue(self) -> str: ...
+    def close(self) -> None: ...
+    @property
+    def closed(self) -> bool: ...
+    def flush(self) -> None: ...
+    def isatty(self) -> bool: ...
+    def read(self, size: int = ...) -> str: ...
+    def readline(self, size: int = ...) -> str: ...
+    def readlines(self, hint: int = ...) -> List[str]: ...
+    def seek(self, offset: int, whence: int = ...) -> None: ...
+    def tell(self) -> int: ...
+    def truncate(self, size: int = ...) -> Optional[int]: ...
+    def __iter__(self) -> 'OutputType': ...
+    def next(self) -> str: ...
+    def reset(self) -> None: ...
+    def write(self, b: Union[str, unicode]) -> None: ...
+    def writelines(self, lines: Iterable[Union[str, unicode]]) -> None: ...
+
+@overload
+def StringIO() -> OutputType: ...
+@overload
+def StringIO(s: str) -> InputType: ...