eolian: enable inheritance checks and fix EFL to build with them

eolian: enable inheritance checks and fix EFL to build with them

Somehow, there was code in the tree that apparently isn't tested at all, even
once - if it was, the eo.c logic that performs inheritance checks would be
triggered. I don't know how this could have happened (actually I do, it's
Cedric's fault and he should be publicly shamed for it) but these checks
make sure this will never happen again. But since the code itself appears
to be untested, I don't know if there isn't any other brokenness in it.
But that's beyond the scope of this change, so for now, let's make sure
all our inheritance is at least formally correct.

Also, enable eo_interface.eo generated code in Eo itself so that Eo.Interface
can be used when inheriting.

@fix