Consider duplicating sync/async code vs relying on `async_to_sync` from asgiref

[joshuadavidthomas]

Initially used the methods from asgiref to cut down on the duplication needed to maintain sync and async flavors of the code, but I have run into issues in implementing and testing #90 that I'm starting to think it would just be simpler to have the code duplicated instead of relying on async_to_sync. On top of that, the sync version then wouldn't have the overhead of dealing with event loops and coroutines when they aren't used at all. (I'm sure the perf overhead is negligible, but still...)

[joshuadavidthomas]

One thing to note is Django's built in manager methods (starting around these lines https://github.com/django/django/blob/main/django/db/models/query.py#L493-L495) define the sync logic and then call sync_to_async for the sync versions, an inversion of our approach in this library. It's that way because gidgethub is async only, so that came first for this library.