This container is type safe and contains a few nice optimizations,
although they shouldn't make a big difference here in practice. The
hashing now uses our default hashing method which reduces code
complexity and seems to perform slightly better in my tests.
For a Heist shot with a highly complex library overrides hierarchy in
the Outliner this reduces the tree building time from around 25 to 23.6
seconds here. However the main design change for performance is yet to
come, all this is just general code refactoring (which at least
shouldn't make performance worse).
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