feat: implement callback_result annotation

[austinabell]

• Implements a callback_result annotation which allows handling errors of promise callbacks
• Adds callback_unwrap annotation to replace callback
  • Given the way that the code is set up, I don't think there is a way to deprecate this callback annotation, but would be happy if someone proves me wrong
• Adds a new example that uses this and handles some use cases not covered in other examples (like stateless methods)

Closes #155

Optionally can add a CallbackResult type which is just

type CallbackResult<T> = Result<T, PromiseError>;

But it's a bit awkward because it really should be PromiseResult, but that type is already built into the current API and is a custom enum

[ChaoticTempest]

just making sure, but would we need to add in callback_unwrap and callback_result to near-sdk-macros/lib.rs? I see there's callback and callback_vec defined there as marker attributes, but I'm not how these relate at all since everything should be going thru the near_bindgen macro itself

[austinabell]

just making sure, but would we need to add in callback_unwrap and callback_result to near-sdk-macros/lib.rs? I see there's callback and callback_vec defined there as marker attributes, but I'm not how these relate at all since everything should be going thru the near_bindgen macro itself

No, that was an artifact of a previous version. These are no longer used anymore, as the annotations now are filtered out of the macro output. I just didn't remove the old ones yet since I'm not sure the implications and didn't want to group that change with this.

[ChaoticTempest]

this looks good to me so far, but might want another set of eyes on this just in case I overlooked anything

[austinabell]

this looks good to me so far, but might want another set of eyes on this just in case I overlooked anything

Yeah, I'm in no rush to get this in, so will leave until it has another review. It is a little more involved of a PR so this is definitely a good candidate for more reviews