[red-knot] Use itertools to clean up `SymbolState::merge` (#15702)
[`merge_join_by`](https://docs.rs/itertools/latest/itertools/trait.Itertools.html#method.merge_join_by)
handles the "merge two sorted iterators" bit, and `zip` handles
iterating through the bindings/definitions along with their associated
constraints.
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Co-authored-by: Micha Reiser <micha@reiser.io>
