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
Support for n-ary UDFs in the parser backend
Support n-ary UDFs in the grammer
Fix unpacking of emit arguments for n-ary UDF
Test of n-ary UDF
Test n-ary UDF with bad column count
Test with nested tuple-valued UDFs
Test with illegal wildcard inside n-ary UDF
typos
Merge pull request #351 from uwescience/whitaker_nary_udf

N-ary UDF functions in myrial
Merge branch 'master' into myria-unique-aggs
Merge pull request #349 from uwescience/myria-unique-aggs

Myria unique aggs
Merge branch 'master' into avg-stdev-cast-double