Create binding specs based on C decls, not Hs decls

[edsko]

(Mostly just a placeholder ticket for now.) This will involve

• Introduce instance resolution pass in the frontend #1447
• Remove the entire Origin datatype

and possibly more.

[edsko]

@TravisCardwell I think we should do this soon-ish; having now seen most the entire codebase while doing #1370, this is the part that stands out to be as most needing refactoring.