Create a class to represent tuple-valued expressions
Instantiate a TupleExpression for UDAs with multiple emitters
Instantaite an NaryEmitArg for tuple-valued UDAs
Check for nested tuple-valued expressions
Fix and test list syntax for UDA emitters
Fix handling of UDAs with multiple return values
Add an extra column to multiple emitters test
Test with non-trival emit expression for tuple-valued UDA
Add apply and walk functions to UdaAggregateExpression
Simplify logic for unpacking tuple-valued expressions
Support n-ary column name lists
Test of n-ary UDA with user-provided column names
Test of bad column name list length
Flake8 fixes