Support anonymous subtypes

[dsaf]

Syntax can be very similar to #13 but I think it should be extended to subtyping other classes as well.

Aside from general usefulness it will prevent potential interface overuse that could happen if only interfaces are supported via #13. I tend to use both interfaces and abstract classes depending on "A is B" vs "A supports B" semantic choices rather than making everything an interface "because DI".

http://www.jot.fm/issues/issue_2005_07/article1.pdf

Note that as generalizations, interfaces compete with abstract classes. In fact, in
practice interfaces and abstract classes are sometimes used as if they were the same
concept. However, since abstract classes have the potential to pass on implementation to
their subclasses, they should be used if (and only if) the relationship to the subclasses is
genetic, i.e., if it is (or at least could be) based on the inheritance of internal structure, that
is, implementation. If on the other hand the relationship is based on pure function (or,
weaker still, on sameness of protocol), interfaces should be used. For instance, a linked
list and a dynamic array would normally not be genetically related (i.e., have no common
pieces of implementation), yet they share the interface of lists (specifying sequential
access to their elements).

I think Java supports this as "anonymous classes".

[svick]

This seems to be a duplicate of #13, only with different (and IMO worse) syntax.

[dsaf]

Actually the syntax was very similar, but yeah it was a half-duplicate.

[gafter]

Closing as a dup of #13.

Noted that @dsaf would like it extended to other types as well.