deconstructing OS

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

We've been slowly decomposing the OS module that's built by mirage-platform 
into separate libraries for a while now, but I've taken the plunge and created 
three experimental repositories for 0.9.9:

- mirage-types : https://github.com/mirage/mirage-types

This contains a single set of module types that build without any dependencies. 
 Right now they contain just IO_PAGE and CLOCK. The intention is that they can 
be satisfied by a specific implementation, so an application can functorize 
over these to be really portable.  It puts them all under a V1 module to permit 
future enhancements.

- io-page : https://github.com/mirage/io-page

This is the standard io_page from mirage-platform, pulled into a separate 
library.  The lib_test directory has a portability test that attempts to cast 
the module into V1.IO_PAGE (and this did in fact find a missing function in the 
Xen version vs the Unix version).

- mirage-clock : https://github.com/mirage/mirage-clock

Just the simple clock functions from mirage-platform, against cast against 
V1.CLOCK

An important part of these libraries is that they can build independently of 
Mirage-platform, so a normal UNIX app can just go ahead and use Io_page without 
having to ever worry about Xen things.

Thoughts?  This split will leave very little in Mirage-platform (just the Time 
and Main modules), along with the runtime libraries, but result in more 
repositories.  I think OPAM mostly takes care of the latter problem however...

-anil