Add `splitfanout` pass #1512

Job	Run time
pre_docs_job	7s
pre_job	7s
Reusable build (ubuntu-latest)	15m 41s
Reusable build (macos-latest)	7m 17s
Try build docs (html)	1m 3s
Try build docs (latexpdf)	56s
Run tests (ubuntu-latest)	10m 58s
Run docs tests (ubuntu-latest)	57s
Run tests (macos-latest)	10m 3s
	47m 9s

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