GC Unchecked types

[sinui0]

This PR decouples the GC message layer from data validation. I've introduced "unchecked" types which are structurally correct but have not been validated against a circuit specification.

Note that I did not invest much into optimizing validation, and we still need more comprehensive tests. I'm punting both to a future PR.

This PR depends on #127 so the diff will be a bit cluttered until we get that merged and I rebase this.