Compatibility with the Mirage's V1_LWT.FS signature

• Can consume Mirage's V1_LWT.FS signature to generate a
  persistent store. This allows to store Git repos directly
  inside raw block devices (no need of filesystem anymore).
• Minor API refactoring to abstract the unix layer cleanly
• Expose a filesystem functor to create filesystem backends
  independent of unix
• Simplify the ocamlfind packages: there's only git and git.unix