There should be no functional change, but this splits up the control
flow across functions, using class fields to hold the state that's
being tracked. The result should be a bit easier to read.
This is part of work to add bind support, but I'm doing some
refactoring in the hierarchy pass to make the code a bit easier to
work with. The idea is that (eventually) the IFExpander object will
hold all the logic for expanding interfaces, and then other code can
do bind insertion.
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