API: Remove `SigningResult`

[woodruffw]

Per discussion in #804: SigningResult is now mostly an internal duplication of a Bundle, so we should just use Bundle directly.

This will be a breaking API change, so slating for 3.0.

[tetsuo-cpp]

@woodruffw I can do this just before we cut 3.0. I assume we wouldn't want this in immediately since it would introduce breaking changes to any point releases that we need to make between now and then.

[woodruffw]

Makes sense to me!

[woodruffw]

#860 is making this a little more pressing, so I'll do this today.