Add support for Irmin 2.4.0 Clear is not implemented for all stores, throwing instead. Arguably it only makes sense for RW stores which have references; content-addressable and append-only don't. Irmin 2.3.0 would also work if there wasn't an incompatibility with ppx_cstruct. There haven't been compatibility breaks since, and Irmin 2.7.2 also works.