Make CallSiteDescriptor a concrete class. It will simplify both lookup handling and class hierarchy a lot.

We can add a "protected final getLookupPrivileged()" method for privileged access to the Lookup object, as well as a "protected final getLookup()" that enforces the security check. We no longer need AbstractCallSiteDescriptor or SimpleCallDescriptor, as we can just collapse their functionality into CallSiteDescriptor itself. equalsInKind and other unnecessary API surface also goes away.