pa_cstruct first cut available

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

I've knocked together the C-struct syntax extension, and it's actually quite 
simple and effective.

https://github.com/avsm/ocaml-cstruct/blob/master/lib_test/pcap.ml

is a pcap parser that cracks open pcap/ethernet/ipv4/tcp and displays the 
payloads. The 'cstruct' declaration defines all the accessors as you might 
expect, and the underlying buffers are all reference counted (as in MPL) and so 
can be passed around. The base-pointer can also be retrieved, so you can do 
comparisons of the underlying Io_page (e.g. to get a Xen grant onto it).

If noone has any objections, I'll add unit tests for the write path, and merge 
this into Mirage in my os-bigarray branch.  I dont think we actually need 
bitstring at all anymore.

Mort: reckon you can easily port the DNS code to this? It should speed up the 
responses an awful lot.

-anil