lib_storage: upgrade to Irmin 2.3.0

Modifies lib_storage/context.ml to use Irmin 2.3.0, which is scheduled for release today. This requires a corresponding change to the opam-repository, but for now those packages are (temporarily) vendored in vendors/dev/.

The only significant change to lib_storage is that read-only store handles must now explicitly sync to avoid stale reads, which is done in the case of exists and checkout.