myrialang: assert that all aggregators are unique when compiling
SimpleGroupBy: remove scheme calculation until it's needed

Do not compute it if we can short-circuit it
rules: add DedupGroupBy
myrialang: use DedupGroupBy
Merge branch 'master' into myria-unique-aggs
Merge pull request #349 from uwescience/myria-unique-aggs

Myria unique aggs