Only make types `Partial` if it's necessary

[sdboyer]

Currently, the default const cuetsy generates for all interface-kinded translated symbols is unconditionally marked as Partial<>. This was, without question, me cutting what appeared to be a cuttable corner, because AFAICT there's no impact on behavior to doing it unconditionally.

Still, it'd be nice to only generate Partial<> defaults if it's actually necessary to do so. And it shouldn't (?) be terribly difficult, as i think the concept corresponds pretty closely to CUE's IsConcrete().