6.0.0

CHANGES:

• Use ptime directly for RO.last_modified, instead of the int * int64 pair
  (#34)
• Add RW.allocate to allocate a key and fill it with zero bytes (#34)
• RO.list: return Key.t instead of string (#37, fixes #33)
• Introduce a custom error for RW.rename with a source which is a prefix of
  destination (#37, fixes #31)
• Use Optint.Int63.t for RO.size, RO.get_partial, RO.set_partial (#37, fixes #32)
• Remove RW.batch (#37, fixes #29 #36, discussed at the MirageOS meeting in
  November, and on the mirageos-devel mailing list in January 2022)
• Key.pp: escape the entire string, not individual fragments (#35)