incgen: flatten process field name references.

* da/compiler/incgen.py:

  -(init): revert old behavior: assign "self.id" instead of "self" to
   'SELF_ID'.

  -(IncInterfaceGenerator.visit_SelfExpr): removed, 'self' reference shouldn't
   appear in inc file.

  -(IncInterfaceGenerator.visit_AttributeExpr): new method, generate "SELF_ID"
   for "self.id", flattens other "self.XX" references.