[CHANGES]
- Added entirely new extension API based on structs of hook functions
registered with the module; similar to Object::Pad and XS::Parse::*
- Provide Future::AsyncAwait::ExtensionBuilder in a similar fashion
to above
- Added some more internal assert() and TRACEPRINT calls for
debugging
[BUGFIXES]
- Don't crash if an async sub's returning future is abandoned outside
of an await state