feat: Add support for user-defined builtins with Dune plugins #611

Job	Run time
install (ocaml-base-compiler.4.12.1, ubuntu-latest)	6m 13s
install (ocaml-base-compiler.4.13.1, ubuntu-latest)	6m 4s
install (ocaml-base-compiler.4.14.0, ubuntu-latest)	6m 2s
install (ocaml-variants.4.14.0+options,ocaml-option-flambda, ubuntu-latest)	6m 21s
install (ocaml-base-compiler.5.0.0, ubuntu-latest)	5m 46s
install (ocaml-base-compiler.5.1.1, ubuntu-latest)	5m 46s
install (ocaml-base-compiler.5.2.0, ubuntu-latest)	5m 49s
install (ocaml-base-compiler.4.12.1, macos-latest)	6m 38s
install (ocaml-base-compiler.4.13.1, macos-latest)	6m 50s
install (ocaml-base-compiler.4.14.0, macos-latest)	6m 32s
install (ocaml-variants.4.14.0+options,ocaml-option-flambda, macos-latest)	7m 24s
install (ocaml-base-compiler.5.0.0, macos-latest)	6m 41s
install (ocaml-base-compiler.5.1.1, macos-latest)	6m 35s
install (ocaml-base-compiler.5.2.0, macos-latest)	6m 18s
install (ocaml-variants.4.12.1+mingw64c, windows-latest)	9m 33s
install (ocaml-variants.4.13.1+mingw64c, windows-latest)	9m 32s
install (ocaml-variants.4.14.0+mingw64c, windows-latest)	9m 40s
install (ocaml-variants.4.14.0+flambda+mingw64c, windows-latest)	10m 14s
	2h 7m 58s

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