#2 interface extension

[Lieberherr]

To make DIs more useful, we would like to support
extension so that we can "grow" DIs incrementally.
A syntax like this might be useful:

di DiName1 with AspectName* extends DiName2
{ClassDictionary}

Extension allows us to extend all building blocks:
interface class graph, strategies, constraints and
visitor methods.

Extensions should be type-safe: they cannot create
run-time type errors.

A motivating example would be a compiler extension:
We add a new feature to a programming language which
requires an update to the interface class graph. This
might trigger some extensions to the traversal strategies
as well as new visitor methods to implement the feature.

Related topics: virtual classes, family polymorphism.