Merge pull request #771 from yallop/fix-ctypes-top #226

Job	Run time
Tests (ubuntu-latest, 4.03.0)	9m 55s
Tests (ubuntu-latest, 4.04.0)	10m 6s
Tests (ubuntu-latest, 4.05.0)	10m 28s
Tests (ubuntu-latest, 4.06.0)	10m 54s
Tests (ubuntu-latest, 4.07.0)	8m 36s
Tests (ubuntu-latest, 4.08.0)	7m 13s
Tests (ubuntu-latest, 4.09.0)	7m 6s
Tests (ubuntu-latest, ocaml-variants.4.11.2+fp+flambda)	7m 52s
Tests (ubuntu-latest, 4.11.1)	7m 20s
Tests (ubuntu-latest, 4.12.0)	7m 1s
Tests (ubuntu-latest, 4.13.1)	6m 47s
Tests (ubuntu-latest, 4.14.0)	6m 51s
Tests (ubuntu-latest, 5.0.0)	7m 12s
Tests (ubuntu-latest, 5.1.0)	7m 33s
Tests (ubuntu-latest, 5.2.0)	7m 30s
Tests (windows-latest, 4.13.1)	7m 36s
Tests (macos-latest, 4.13.1)	5m 58s
	2h 15m 58s

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