Better handling of "fixed" public inputs in CIVC

[ledwards2225]

In CIVC there are several distinct objects that are stored in the public inputs in "fixed" sense, e.g. pairing points, IPA accumulator, Databus commitments, (and eventually ecc op queue table commitments for the merge protocol). The storage of these components (i.e. metadata about their size and location within the public inputs) is clunky and handled in multiple inconsistent ways. It would be nice to have some kind of PublicInputs class that makes the handling uniform and provides a single location/object to understand what's being propagated.