checkInputs used to be added to nativeBuildInputs. Now we have
nativeCheckInputs to do that instead. Doing this treewide change allows
to keep hashes identical to before the introduction of
nativeCheckInputs.
* ocamlPackages.yaml: 2.1.0 -> 3.0.0
Also added the checkInputs.
* Add ocamlPackage.yaml-sexp 3.0.0
The part of yaml that requires sexplib is moved to a second package.