eolian: add API to query information about Objects

eolian: add API to query information about Objects

As nearly every Eolian handle is backed by an Eolian_Object,
this information is now publicly exposed and has an API. This
opens up an array of new possibilities for tooling, as you
can now externally query file names, line numbers etc.,
as well as cast arbitrary handles to Eolian_Object pointers
and back.

This will be expanded later and it will replace the Declaration
system, as it's cleaner, better integrated and more versatile.

@feature