This change refactors the subquery code into a separate builder class to
help allow for more reuse and make the functions smaller and easier to
read.
The previous function that handled most of the code was too big and
impossible to reason through.
This also goes and replaces the complicated logic of aggregates that had
a subquery source with the simpler IteratorMapper. I think the overhead
from the IteratorMapper will be more, but I also believe that the actual
code is simpler and more robust to produce more accurate answers. It
might be a future project to optimize that section of code, but I don't
have any actual numbers for the efficiency of one method and I believe
accuracy and code clarity may be more important at the moment since I am
otherwise incapable of reading my own code.
Jonathan A. Sternberg