6.0.1

CHANGES:

• Fix OCaml documentation strings (#30 @hannesm)
• Add caml_get_wall_clock to retrieve the wall clock as int64 (#31 @hannesm)
• Don't require opam for the bindings compilation (#32 @sternenseemann)
• Remove checksum_stubs and alloc_pages_stubs (available in mirage-tcpip
  and io-page), to keep in sync with mirage-solo5 (#33 @dinosaure, adapted
  by @hannesm to mirage-xen)