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This does two things: * Eliminate all uses of Unique.deriveUnique, which was quite easy to mis-use and extremely subtle. * Rename the previous "derived unique" notion to "local unique". This is possible because the only places where `uniqAway` can be safely used are those where local uniqueness (with respect to some InScopeSet) is sufficient. * Rework the implementation of VarEnv.uniqAway, as discussed in #17462. This should make the operation significantly more efficient than its previous iterative implementation.. Metric Decrease: T9872c T12227 T9233 T14683 T5030 T12545 hie002 Metric Increase: T9961
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