Fixed context ranges

As the python parser does not provide "end" information, we need to go up the parent chain when encountering, for example, a statement in a function definition, and adjust the ending positions of all parent nodes (especially the function definition).

Otherwise, the context cannot be created correctly. This has been a problem before, but reappeared when we switched to the new python parser.

It should be gone now, anyway, together with the ugly "body.last()", "body.first()" hacks (the FunctionDefinitionAst node now has proper start and end attribtutes set).