Add `splitfanout` pass #1531

Job	Run time
pre_docs_job	5s
pre_job	6s
Reusable build (ubuntu-latest)	16m 6s
Reusable build (macos-latest)	7m 49s
Try build docs (html)	1m 10s
Try build docs (latexpdf)	56s
Run tests (ubuntu-latest)	10m 31s
Run tests (macos-latest)	8m 52s
Run docs tests (ubuntu-latest)	25s
	46m 0s

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