spin off of #11549.

http://bugs.python.org/issue11549#msg130955

> b) Docstring is now an attribute of Module, FunctionDef and ClassDef, > rather than a first statement. Docstring is a special syntactic 
> construction, it's not an executable code, so it makes sense to separate it. Otherwise, optimizer would have to take extra care not to introduce, change or remove docstring. For example:
>
>  def foo():
>    "doc" + "string"
>
>Without optimizations foo doesn't have a docstring. After folding, however, the first statement in foo is a string literal. This means that docstring depends on the level of optimizations. Making it an attribute avoids the problem.