Functoria release

The mirage tool is now based on functoria.
See https://mirage.io/blog/introducing-functoria for full details.

• Command line interface: The config file must be passed with the -f option
  (instead of being just an argument).

• Two new generic combinators are available, generic_stack and generic_kv_ro.

• get_mode is deprecated. You should use keys instead. And in particular
  Key.target and Key.is_xen.

• add_to_ocamlfind_libraries and add_to_opam_packages are deprecated. Both
  the foreign and the register functions now accept the ~libraries and
  ~packages arguments to specify library dependencies.

• If you were using tls without the conduit combinator, you will be
  greeted during configuration by a message like this:

  The "nocrypto" library is loaded but entropy is not enabled!
  Please enable the entropy by adding a dependency to the nocrypto device.
  You can do so by adding ~deps:[abstract nocrypto] to the arguments of Mirage.foreign.
  

  Data dependencies (such as entropy initialization) are now explicit.
  In order to fix this, you need to declare the dependency like so:

  open Mirage
  
  let my_functor =
  let deps = [abstract nocrypto] in
  foreign ~deps "My_Functor" (foo @-> bar)

  My_functor.start will now take an extra argument for each
  dependencies. In the case of nocrypto, this is ().

• Remove nat-script.sh from the scripts directory, to be available
  as an external script.