Implement a LOOKUP-TABLE JOIN function

There are many situations (e.g., scoring, tagging with percentiles), when you have a table of values with an ascending/descending key sequence (e.g., score), and you want to annotate records with the first match that satisfies a condition.

You can do it at the moment with a filtered project. E.g.,

myLookupDataset(score > myThreshold)[1].....

but that isn't efficient for large datasets, and we should be able to generate a more efficient implementation.

Triggered by a discussion of the new QUANTILE function.