eolian: add a new references system to help replace pointers

eolian: add a new references system to help replace pointers

It's now possible to mark struct fields and function params as "references",
which causes them to become pointers in C (in bindings, they become whatever
is necessary). They're not a part of the type and are much more restricted
than pointers, allowing bindings to be easier. This system will be gradually
utilized and expanded as required.

@feature