Simplify and Streamify `EventSubscription`

[jsdw]

I reckon we could improve/simplify the EventSubscription struct. Some thoughts:

• Impl Stream (and so the StreamExt methods), so that the filtering logic can be (mostly?) moved out of this struct.
• Remove filtering on events from a specific block (if needed, add a separate method to obtain events from a single block which potentially returns the same event stream as we'd get here; this method could be reused in the transaction.rs stuff too if it existed).
• Remove filtering of events from a specific extrinsic (we get this now via the API in transaction.rs, and ideally it would be easy to manually filter the Stream for such events).
• Remove filtering of events by pallet/event index; make it easy to do so via the Stream.
• Update transfer_subscribe example to subscribe to events separately from making any transaction, and show off filtering and such. If needed, make transactions etc in a separate thread just to produce the desired events. (why? we shouldn't use this to get events for the current transaction as we have a proper API for that; but that's what the example seems to do).
• Possibly add other examples.

[paulormart]

Just read this issue and PR #423 and i'am fully supportive for the implementation of this feature. This is definitely a must have for this library :)

I'm currently using a custom events subscription so that block number and authority index could be easily accessible here

[jsdw]

Given your feedback and the PR, I think I'll bump this up my todo list and start working on it fairly soon!

[jsdw]

@paulormart , @lamafab and anybody else interested, feel free to check out #442 and leave feedback :)