consider making a defaultdict-like wrapper for OrderedDict

[cosmicexplorer]

In #7116 we add union rules to the RuleIndex, which involves manually checking if an entry is in an OrderedDict, and if so initializing it. It doesn't seem super hard to make a quick wrapper type which would do this if this is something we end up doing more often with OrderedDicts. It's possible this technique could be generalized to other collections.

[jsirois]

I'm going to close this since defaultdict is documented as extending dict and even though in Python 3.6 dict preserving insertion order is a CPython implementation detail, we are wedded to CPython via Rust FFI so we can rely on this detail in Python 3.6. We no longer have to worry about this in Python 3.7+ where dict is documented as preserving insertion order.