Add `splitfanout` pass #1511

Job	Run time
pre_job	7s
pre_docs_job	7s
Reusable build (ubuntu-latest)	15m 30s
Try build docs (html)	59s
Reusable build (macos-latest)	9m 40s
Try build docs (latexpdf)	58s
Run docs tests (ubuntu-latest)	29s
Run tests (ubuntu-latest)	10m 43s
Run tests (macos-latest)	9m 5s
	47m 38s

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