Replace implementation with Mirage one

This exposes both the high-level interface (share_pages etc) and the
low-level interface (grant_access etc).

The style is the same as io-page, specifically there are 2 ocamlfind
packages:

xen-gnt: contains the pure OCaml code which references external C stubs
xen-gnt.unix: contains the userspace C stubs

eventually we will have

xen-gnt.xen: contains the kernelspace C stubs

It's up to the application to link in the correct C stubs (it's the C
equivalent of a functor)

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Regenerate OASIS

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update ChangeLog

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Fix 'make uninstall'

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #1 from djs55/update

Tentative: share grant table code with Mirage