eolian: add support for composite section into regular classes
Each regular class can now have a section called 'composite',
which can contain interfaces and interfaces only. This defines
a list of interfaces that are allowed to be unimplemented on the
class, as it is assumed the class will be composited with some
other class implementing those interfaces.
Only regular classes can have this, as only regular classes can
It will also be necessary to check whether the classes in the
section appear somewhere within the inheritance tree. For now,
this testing is not being done.
Example of usage:
directly in the class body.