split the `mirage-xen` package into 3

• Split mirage-xen into three opam packages: mirage-xen-posix (includes and mini-libc), mirage-xen-ocaml (OCaml runtime) and mirage-xen (bindings and OCaml OS libraries) (#125, patch from @hannesm)

Add opam files and remove `page_stubs.c`

• Add opam files for mirage-xen and mirage-unix OPAM packages
• Remove page_stubs.c, now provided by io-page (#122, patch from @hannesm)

Add generic engine hooks and restore GC tracing support

• Add generic hooks to mainloop to support background tasks. (#120)
• [xen] Report trace events for GC again; this was disabled temporarily
  in the 2.2.0 release. (#119)

Fix modern GCC and Xen compilation

Fix Xen compilation with gcc 4.8+ by disabling the stack protector and -fno-tree-loop-distribute-patterns (not compatible with MiniOS). These missing flags were a regression from 2.1.3, which did include them.

Add support for OCaml 4.02+ in Xen

This releases adds support for OCaml 4.02+ compilation, and changes the Xen
backend build for Mirage significantly by:

• removing the OCaml compiler runtime from the mirage-platform, which makes
  it simpler to work across multiple revisions of the compiler. It now uses
  the ocaml-src OPAM package to grab the current switch's version of the
  OCaml runtime.
• split the Xen runtime build into discrete pkg-config libraries:
  • mirage-xen-posix.pc : in the xen-posix/ directory, is the nano-posix
    layer built with no knowledge of OCaml
  • mirage-xen-minios.pc: defines the __INSIDE_MINIOS__ macro to expose
    internal state via the MiniOS headers (for use only by libraries that
    know exactly what they are doing with the MiniOS)
  • mirage-xen-ocaml.pc: in xen-ocaml/core/, this builds the OCaml asmrun,
    Bigarray and Str bindings using the mirage-xen-posix layer.
  • mirage-xen-ocaml-bindings.pc: in xen-ocaml/bindings/, these are bindings
    required by the OCaml libraries to MiniOS. Some of the bindings use MiniOS
    external state and hence use mirage-xen-minios, whereas others
    (cstruct_stubs and barrier_stubs are just OCaml bindings and so just
    use mirage-xen-posix).
  • mirage-xen.pc: depends on all the above to provide the same external
    interface as the current mirage-platform.

The OCaml code is now built using OASIS, since the C code is built entirely
separately and could be moved out into a separate OPAM package entirely.

Improved reporting of top-level errors in Xen backend

• [xen] Fix error handling in OS.Main.run to enable a top-level
  exception to signal to the Xen toolstack that it crashed (versus a
  clean exit). This in turn lets on_crash="preserve" behaviour work
  better in Xen VM description files.
• Remove mirage-xen.pc file on uninstall.

Further Xen/MiniOS header cleanup

[xen] Updated headers and build for Mini-OS 0.5. This involves:

• Require libminios >= 0.5
• Remove old includes directory when installing
• Compile with -fno-builtin (avoids warnings about standard functions)
• Removed complex.h (now provided by Openlibm)
• Include cdefs.h from types.h (needed for __BEGIN_DECLS)
• Removed open from unistd.h (comes from fcntl.h)
• Removed assert.h and __assert_fail (provided by Mini-OS)
• Removed string.h (provided by Mini-OS)
• Removed cdefs.h (provided by Mini-OS)
• Added missing console.h includes (for printk)

Add header definitions for Ctypes support

• Remove checksum stubs from Unix and Xen, as they are provided by tcpip now.
• [xen] Define UINTx_MAX and SIZE_MAX in stdint.h, which is sufficient to let Ctypes compile (yallop/ocaml-ctypes#231)

Add Xen tracing and improve packaging of C runtime

• [xen] Report trace events for GC, block_domain, XenStore and event channels.
  This introduces a new dependency on the mirage-profile package.
• [xen] Install a pkg-config file to allow other projects to compile C stubs
  against mirage-xen.
• [xen] Remove duplication of OCaml header files inside the include tree.

Xen stability: use monotonic time, check for page aligned I/O and add assert stubs

• [xen] Assert that pages passed to the grant share API are page-aligned.
  This always happens if they are created via Io_page.create, and
  probably not true if made by Cstruct.create.
• [xen] Use monotonic time for timing events, not wall-clock time.
• [xen] Provide functions that C code often uses for asserts (abort,
  printf, etc).