Add after_<type> callbacks

[marcandre]

I double-checked that this has basically no performance impact (less than ~0.1% overall) after #8785 (otherwise that would add ~4-5% overall)

[bbatsov]

Won't after_something be a simpler name? on_after seems a bit confusing to me. :)

[marcandre]

Won't after_something be a simpler name? on_after seems a bit confusing to me. :)

I liked that it made it clear what where the callbacks: all methods starting with on_. I can check how more complicated the code is to handle after_.

[bbatsov]

Yeah, I got why you named it this way, but the only reason why all callbacks started with on_ is that they are all the same type of callbacks. I also believe it's quite uncommon to combine two prepositions in a name. I'm not that concerned about the naming per se, rather about people understanding what the new callbacks are about.

[marcandre]

Changed on_after => after. It is definitely better 👍

@bbatsov May I merge this now, or would you rather wait for 1.0 to be out?

[bbatsov]

@marcandre Merge it! 🚀