eolian: make doc ref resolution global

eolian: make doc ref resolution global

Doc refs no longer introduce new dependencies into files. Instead,
they're parsed globally, and any doc ref lookup is also made
globally. This allows unit based dependencies to correspond more
to what files actually really need at compile time/runtime, with
docs being irrelevant to that; it also simplifies the API.

The doc resolution API now takes Eolian_State instead of
Eolian_Unit, too.