ENH: Tooling for debugging backends #11

While tracing helps with debugging and one can print dispatchable._implementations, we need some light tooling to debug backends. E.g. this might be a SPATCH_DEBUG_BACKENDS=... env variable (possible with multiple settings):

1. Greedily import all type identifiers immediately. No-import name matching is great, but it means that misspelling a module or type name can lead to a backend silently never matching.
2. A way to see that a function was (and maybe was not) bound to a dispatchable (i.e. if there are unused ones).
   • A problem with finding ones that are unused is that you need to know that all dispatchables were created already and this is tooling spatch cannot provide. I still think that both would be very valuable, even we decide to solve this via a pattern that libraries should provide.