Remove `Core` variants of various types

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

Start by locating the listed Core types throughout the Rust crate and compare each with its corresponding non-Core API, beginning with StreamCore and Stream. Trace how container types and their contents are represented across the affected APIs. Done means all listed Core variants are removed or made the default while preserving the intended container abstraction as a breaking API change.