Right now if we want an O(1) header it has to be added to the core header map. This is unfortunate for extension writers who might want this perf boost but either can't add it due to private code, or as we move away from having extensions modify the core code in any way, will be blocked from adding it.

I believe it should be possible to allow extension O(1) headers to be added. A rough implementation outline is:

1. Allow the struct of O(1) headers to be variable sized and zero initialized.
2. Each O(1) header is statically registered with a handle and a slot index into the array, as well as the lookup trie.
3. Allow extensions to statically register and get their own slot.