Add dynamic inner classes

[peq]

Like #213, but also dynamic inner classes.

[peq]

[Note to self]

Open questions:

• Store only one reference to immediate enclosing class or a pointer to all or do it like with closures and actually check what is required?
• Do we need something like x.new MyClass() as in Java?
• Should instances of inner classes be destroyed automatically when destroying the parent object? (probably too expensive to keep a list of children)
• Optimize the case where there can be at most one child? In this case it would be possible to share ids.