eolian: add API to retrieve the C name of an object

eolian: add API to retrieve the C name of an object

This is to prepare for type/class renaming support. This adds
the necessary API to retrieve C-specific names. Other refactoring
is necessary elsewhere for now.

This also renames the old API eolian_class_c_name_get to
eolian_class_c_macro_get to avoid conflict as well as clarify
the intention.