Define struct cheri_object, consisting of a pair of code and data
capabilities, to be used within struct sandbox_class rather than having
it reference them separately, and for direct use in sandboxes.
Hook up libc_cheri to the build -- this library will be a subset/adaptation
of the system libc suitable for use in sandboxes.  Right now it contains
only the function cheri_invoke() to allow invoking services outside of the
sandbox.