make some things guaranteed to be deterministic (#1065)
This PR replaces a few uses of hash maps/sets with btree maps/sets and
index maps/sets. This has the benefit of guaranteeing a deterministic
order of iteration.
I made these changes as part of looking into a flaky test.
Unfortunately, I'm not optimistic that anything here will actually fix
the flaky test, since I don't believe anything was actually dependent
on the order of iteration.
