Interface cleanups and entropy support

• Remove the IO_PAGE module type from V1. This has now moved into the
  io-page pacakge (#356)
• Remove DEVICE.connect from the V1 module types. When a module is
  functorised over a DEVICE it should only have the ability to
  use devices it is given, not to connect to new ones. (#150)
• Add FLOW.error_message to the V1 module types to allow for
  generic handling of errors. (#346)
• Add IP.uipaddr as a universal IP address type. (#361)
• Support the entropy version 0.2+ interfaces. (#359)
• Check that the opam command is at least version 1.2.0 (#355)
• Don't put '-classic-display' in the generated Makefiles. (#364)