dataflow/operators/core: add `.partition()` for `StreamCore` #664

Job	Run time
test mdBook	40s
cargo test on ubuntu, rust stable	15s
cargo test on ubuntu, rust 1.78	25s
cargo test on macos, rust stable	28s
cargo test on macos, rust 1.78	57s
cargo test on windows, rust stable	1m 49s
cargo test on windows, rust 1.78	52s
	5m 26s

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