Remove wait_for_event and simplify event access in subscribe_events

[haerdib]

Removes the obsolete wait_for_event functions. I don't see the use case of these functions any more.

In addition, the event subscription has been updated:

• Renames subscribe_frame_events to subscribe_events and moved to more fitting event.rs instead of the frame_system file.
• Instead of returning the StorageChangeSet, which required quite some type insight to retrieve the events from it, a struct EventSubscription is returned. Upon next_event it directly tries to retrieve the events.

closes #413

[Niederb]

I like the new EventSubscription. I think it really makes the event extraction easier.
One thing that is not quite clear to me is how the EventDetails and EventRecord relate as they seem quite similar. Is it a possibility to get rid of EventDetails?

[clangenb]

Nice, looks good to me! But we should clarify the drop stuff.

[haerdib]

One thing that is not quite clear to me is how the EventDetails and EventRecord relate as they seem quite similar. Is it a possibility to get rid of EventDetails?

True enough. That might be something one could look into. EventDetails has been copy pasted from subxt. It's basically a helper / wrapper struct. EventRecord is the Substrate type. But that should be investigated in a different issue.

EventDetails does contain quite some convenience features. But maybe it's an overkill. Will create an issue for that.

See: #440

[Niederb]

LGTM. Thanks for the clarification in the comment and the opening of the issue 👍

[clangenb]

Thanks a lot, lgtm now!