Allow out-of-order event registration

[UkoeHB]

We can do this by storing registered events in a sorted list of typeids, then assigning channel ids based on index.

[Shatur]

Agree, but shouldn't use TypeId since it's not stable across binaries. Relevant issue in Bevy. But it looks like we can just use type names.

[UkoeHB]

It looks like type_name is also not stable.

[Shatur]

Hm... Yes, I missed it. That just what @cart suggested in https://github.com/bevyengine/bevy/issues/32#issuecomment-673784868. Then I'm not sure how to solve it properly.