Add macos to the CI #145

Job	Run time
build (ubuntu-latest, 4.13)	4m 32s
build (ubuntu-latest, 4.14)	4m 22s
build (ubuntu-latest, 5.0)	4m 4s
build (ubuntu-latest, 5.1)	3m 57s
build (ubuntu-latest, 5.2)	2m 46s
build (windows-latest, 4.13)	7m 4s
build (windows-latest, 4.14)	6m 52s
build (windows-latest, 5.0)	7m 36s
build (windows-latest, 5.1)	7m 30s
build (windows-latest, 5.2)	5m 39s
build (macos-latest, 4.13)	4m 13s
build (macos-latest, 4.14)	5m 28s
build (macos-latest, 5.0)	4m 26s
build (macos-latest, 5.1)	4m 27s
build (macos-latest, 5.2)	3m 22s
build (ubuntu-latest, 4.08)	4m 26s
build (ubuntu-latest, 4.09)	4m 26s
build (ubuntu-latest, 4.10)	5m 0s
build (ubuntu-latest, 4.11)	4m 42s
build (ubuntu-latest, 4.12)	4m 23s
build (windows-latest, ocaml-base-compiler.4.14.2,system-msvc)	7m 52s
build (windows-latest, ocaml-variants.5.2.0+msvc)	5m 59s
	1h 53m 6s

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