Custom objects returned by modules must be subclass of ModuleObject and
have the state argument in its methods.
Add MutableModuleObject base class for objects that needs to be deep
copied on assignation.
The only advantage they have is they have the interpreter in arguments,
but it's already available as self.interpreter. We should discourage
usage of the interpreter API and rely on ModuleState object instead in
the future.
This also lift the restriction that a module method cannot add build
targets, but that was not enforced for snippet methods anyway (and some
modules were doing it) and it's really loose restriction as it should
check for many other things if we wanted to make it consistent.
Xavier Claessens