change xen installation to place libraries in a separate mirage-xen area, to avoid ocamlfind complaining about subdirectories in a package
add C stubs for the Str module, adapted to remove use of compatibility (e.g. stat_alloc to caml_stat_alloc).

We want to get rid of this eventually, but that can wait for namespaces and such
update CHANGES and bump to 0.8.0