Implement closures and nested classes.

[Eike]

The current implementation of the environment is not smart enough to allow arbitrary nesting of classes and functions. Therefore the environment implementation should be improved.
The current implementation of the environment is not smart enough to allow arbitrary nesting of classes and functions. Therefore the environment implementation should be improved.

Especially interesting are class definitions inside functions, because functions that return classes could mimic parameterized types.
Especially interesting are class definitions inside functions, because functions that return classes could mimic parameterized types.

Problematic is the "this" argument and the automatic lookup of member attributes through it. C++ therefore probably has no class definitions inside functions.