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.
As it is no longer necessary to pass unit when evaluating exprs,
it is not necessary to pass it here either. Convert all the APIs
to the new style and update all instances in our tree.
Daniel Kolesa