• Feature: support for in-memory transactions. They are built
  on top of views.
• Feature: add support for views: these are temporary stores with
  lazy reads + in-memory writes; they can be used to convert back
  and forth an OCaml value into a store, or to have a fast stagging
  area without the need to commit every operation to the store.
• Support custom messages in commit messages
• Improve the IrminMerge API
• Backend: add a 'dispatch' backend for combining multiple backends
  into one. This can be used to have a P2P store where there is
  well-defined mapping between keys and host (as a DHT).
• Fix: limit the number of simulteanous open files in the Git and
  the file-system backend
• Speed-up the in-memory store
• Speed-up the import/export codepath
• Speed-up the reads
• Speed-up IrminValue.Mux
• Deps: use ocaml-sha instead of cryptokit