eolian: add new unit API

eolian: add new unit API

The idea of units (as in translation units) is to encapsulate a single
parser pass into a structure to prevent leakage of data from other
passes (therefore catching error cases that couldn't be caught before).

The internals of the unit system are currently not implemented, this
batch merely switches all the relevant APIs to use units as well as
updates various things that use those APIs; implementation will come
afterwards.

@feature