- rework `make_graphql_filter()` and `make_graphql_filter_sync()` to execute `context_extractor` only once
- handle all non-recoverable `Rejection`s in `make_graphql_filter()` and `make_graphql_filter_sync()`
- relax requirement for `context_extractor` to be a `BoxedFilter` only
- remove `JoinError` from public API
- provide example of fallible `context_extractor` in `make_graphql_filter()` API docs
Additionally:
- split `juniper_warp` modules into separate files
- add @tyranron as `juniper_warp` co-author
