The hand-rolled notion of flow was causing some severe performance
issues (on a few databases):
```
Tuple counts for Pythagorean::square#168e234a#f#loop_invariant_prefix/2@c86989kr after 6m35s:
175000 ~5% {2} r1 = JOIN SSA::SsaVariable::getDefinition#dispred#f0820431#ff_10#join_rhs WITH Flow::ControlFlowNode::getNode#dispred#f0820431#bf ON FIRST 1 OUTPUT Lhs.1, Rhs.1 'arg0'
174500 ~6% {2} r2 = JOIN r1 WITH SSA::SsaVariable::getVariable#dispred#f0820431#ff ON FIRST 1 OUTPUT Rhs.1, Lhs.1 'arg0'
1467782500 ~5% {3} r3 = JOIN r2 WITH AstGenerated::Name_::getVariable#dispred#f0820431#ff_10#join_rhs ON FIRST 1 OUTPUT 3, Rhs.1 'arg1', Lhs.1 'arg0'
1467553000 ~0% {2} r4 = JOIN r3 WITH py_expr_contexts_12#join_rhs ON FIRST 2 OUTPUT Lhs.2 'arg0', Lhs.1 'arg1'
return r4
```
Rewriting it to use the data flow library made all of this go away. 🎉
