Add bulk changes event for collection events

[jcornaz]

It may (and often is) simpler/faster for the consumer of the event to handle bulk changes.

By "bulk" I mean having a single event representing a change of many elements in the collection.

Examples of bulk changes event:

• SublistAppended
• ListTruncated
• SubsetAdded
• SubsetRemoved

[jcornaz]

Should come with operator for simplifying and aggregating.

Examples:

• fun ReceiveChannel<SimpleListEvent<T>>.aggregate(): ReceiveChannel<ListEvent<T>>
• fun ReceiveChannel<ListEvent<T>>.simplify(): ReceiveChannel<SimpleListEvent<T>>