[m-rev.] for review: faster merging of varsets

[Zoltan Somogyi]

For review by anyone.

The diff implements the proposed faster algorithm just once,
even though it is applicable to four predicates. I plan to address
any review comments on that one predicate, and *then* copy
the approach to the other three.

If someone would undertake to run tuning benchmarks for me,
I would write the required benchmarking code. (The compiler
at the moment goes out of its way to avoid calling these predicates
with argument sizes where the old algorithm is too slow,
so I don't think it would be a good benchmark for this.
The main objective of this diff is the avoidance of this need.)

Zoltan.
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