some low-hanging fruit in UM join construction

looking into what is necessary for the supertask. Going to use this subtask to track easy optimizations to be made after the main task is done.

• Unnecessary branching on every row (is it this type, is it that type, etc). TODO: Pass a whole RowGroup into TupleJoiner::insert(), so the branches can be evaluated once for a rowgroup rather than on every row.
• tuplejoiner could use some cleanup in general. this has gotten more hard to understand than is justified...