Demo Gen: Unflatten Terminus Functions

[ambiguousname]

Quick fix in #673, but a long term solution for introducing DiplomatOption<> would be to set up "helper" functions that can be re-used in struct construction or opaque construction.

That way, any type can be labelled as "optional", and thus the constructor that we use for that type can also be labelled as "optional". Then we can mark any swath of parameters as optional in our RenderInfo output.