Remove scope bound example implementations (#819)

Removes ScopeBound... classes to reduce the API surface.

- ScopeBoundCallable: Was intended for event loops. Event loop decorators should instead implement this in a way that fits the specific loop API the best with regard to avoiding unnecessary references to arguments and error handling.
(The implementation was (mostly) sufficient for react/event-loop but shouldn't be used for revolt/event-loop.)
- ScopeBoundPromise: Was only intended as very basic example implementation. An actual implementation has to hook into the specific promise implementation to capture the context of the initial promise.
(I'm not sure how useful promise/future support is in general, there are scenarios where we cannot preserve the context association, e.g. when extracting/awaiting the value.)

Author: Nevay