feat(ocap-kernel): Add kernel syscall 'revoke'

[grypez]

WIP

This PR prototypes a user code authority to effect a kernel syscall which accepts a remotable object argument. This solution mostly bypasses liveslots, extracting vrefs by hacking apart the return values of the only-exported-for-testing marshaller; this probably messes up liveslots's book keeping efforts.

[sirtimid]

Both makeOcapUrl and makeRovoker are synchronous, which was the argument against using a device. I wonder could we pursue that?

[grypez]

Both makeOcapUrl and makeRovoker are synchronous, which was the argument against using a device. I wonder could we pursue that?

With kernel service objects #563 these user-space syscalls are E-addressable from the vats that rely on them. Will update this soon to reflect.