Remove `Core` variants of various types

[antiguru]

We added several Core variants of types to support different container types. For example, Stream<G, D> is really a StreamCore<G, Vec<D>>. We should remove the Core variants and make them the default instead.

This is a breaking change, but it seems we have sufficient experience with the container abstraction that we can make the container type with its contents to the default stream element.

The following types need to be adjusted:

• StreamCore
• TeeCore
• EventProducerCore
• EventConsumerCore
• BundleCore
• ParallelizationContractCore
• ExchangeCore
• BufferCore
• AutoflushSessionCore
• CounterCore
• feedback::HandleCore
• input::HandleCore
• UnorderedHandleCore
• EventIteratorCore
• EventPusherCore
• EventLinkCore
• EventWriterCore
• EventReaderCore
• InputHandleCore
• FrontierdInputHandleCore
• OutputHandleCore