Check whether loans conflict with old loans or with themselves. #3769
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Along the way, convert from dvec-of-dvec representation to track loans in scope
to just a single flattened list. It's more convenient.
Fixes #3765.
I'm actually not sure whether this needs review. I'm inclined to think not, it's a fairly targeted bug fix. But I thought I'd throw it up as a pull request and give people the chance to tell me if they agree. =)