merlin also needs to be updated at the same time because of
incompatibilities between the two versions.
* gnu/packages/ocaml.scm (ocaml-merlin-lib): New variable.
(ocaml-dot-merlin-reader): Inherit from it and update inputs.
(ocaml-merlin): Update inputs.
(ocaml-yojson): Update to 2.0.2.
(ocaml-odoc): Disable tests.