Allow len() with a Tuple argument

Fixes #158.

Author: JukkaL

mypy/test/data/pythoneval.test

@@ -838,3 +838,9 @@ import typing
 'x'.count('x')
 'x'.count(u'x')
 [out]
+
+[case testLenOfTuple]
+import typing
+print(len((1, 'x')))
+[out]
+2

stubs/3.2/builtins.py

@@ -964,7 +964,10 @@ def isinstance(o: object, t: tuple) -> bool: pass
 def issubclass(cls: type, classinfo: type) -> bool: pass
 # TODO perhaps support this
 #def issubclass(type cld, classinfo: Sequence[type]) -> bool: pass
+@overload
 def len(o: Sized) -> int: pass
+@overload
+def len(o: tuple) -> int: pass
 def locals() -> Dict[str, Any]: pass
 
 # TODO more than two iterables