Now that the compiler supports capabilities in structure arguments to
functions being passed via registers, pass around bundles of {code,
data} capabilities using struct cheri_object rather than as individual
pointers, which makes things quite a bit more readable (and less
error-prone).
Replace cheri_systemcap_get() with sandbox_object_getsystemobject(): rather
than using a single object capability for all CHERI system class references,
instead construct per-sandbox object capabilities whose code capability is
the global non-sandbox, but whose data capability is the current sandbox's
sandbox_object structure.

Change consumers to pass in the per-sandbox reference rather than caching
global references.