Opaque origins are meant to be unique in terms of equality from
one another. Since this uniqueness needs to be across processes,
use a nonce to implement the uniqueness check.
Instead, porting over all users to use the newly created
Origin::create_opaque factory function. This also requires porting
over some users of Origin to avoid default construction.
This lets us move a few Host-related functions (like serialization and
checks for what the Host is) into Host instead of having them dotted
around the codebase.
For now, the interface is still very Variant-like, to avoid having to
change quite so much in one go.
