Fix possible quadratic performance bug in Backend.hs #2117
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@runarorama Randomly spotted this. The previous code would build a new relation for each entry in the map, rather than building it just once and doing multiple lookups into the relation.
This would be more noticeable when the query returns many terms and types. I'm not exactly sure how often that would be but still seemed worth fixing.
Laziness might help somewhat but even with laziness, it will have to force a lot of the relation in order to do a
lookupRan. It's also possible that with optimizations turned on that common subexpression elimination hoists this out of the loop but I wouldn't necessarily count on that.