feat: Add support for user-defined builtins with Dune plugins #616

Job	Run time
install (ocaml-base-compiler.4.12.1, ubuntu-latest)	5m 38s
install (ocaml-base-compiler.4.13.1, ubuntu-latest)	5m 51s
install (ocaml-base-compiler.4.14.0, ubuntu-latest)	5m 56s
install (ocaml-variants.4.14.0+options,ocaml-option-flambda, ubuntu-latest)	6m 23s
install (ocaml-base-compiler.5.0.0, ubuntu-latest)	5m 45s
install (ocaml-base-compiler.5.1.1, ubuntu-latest)	5m 51s
install (ocaml-base-compiler.5.2.0, ubuntu-latest)	5m 31s
install (ocaml-base-compiler.4.12.1, macos-latest)	6m 42s
install (ocaml-base-compiler.4.13.1, macos-latest)	7m 5s
install (ocaml-base-compiler.4.14.0, macos-latest)	6m 37s
install (ocaml-variants.4.14.0+options,ocaml-option-flambda, macos-latest)	6m 56s
install (ocaml-base-compiler.5.0.0, macos-latest)	6m 23s
install (ocaml-base-compiler.5.1.1, macos-latest)	7m 54s
install (ocaml-base-compiler.5.2.0, macos-latest)	7m 25s
install (ocaml-variants.4.12.1+mingw64c, windows-latest)	9m 18s
install (ocaml-variants.4.13.1+mingw64c, windows-latest)	9m 34s
install (ocaml-variants.4.14.0+mingw64c, windows-latest)	9m 27s
install (ocaml-variants.4.14.0+flambda+mingw64c, windows-latest)	10m 13s
	2h 8m 29s

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