simplify polonius=next

Remove incomplete handling of kills during traversal for loan liveness
to get to a simpler and actionable prototype.

This handles the cases, on sufficiently simple examples, that were
deferred from NLLs (NLL problem case 3, lending iterators), and is still
a good step to put in people's hands without needing to wait for another
full implementation. This is a practical cut in scope, but it also
shows where are the areas of improvement, that we will explore in the
future.

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