chaperones: allow struct chaperones that supply no redirections

(as requested by Asumu)

A witness accessor or mutator is still required to create a structure
chaperone, but `#f` can be provided in place of a redirection, and
then impersonator properties can be attached to the chaperone.

At the same time, adjust `(chaperone-of? v1 v2)` so that `v1` as a
chaperone is not required to preserve non-redirecting chaperones of
`v2`.

The overall consequence is that a redirection procedure can cooperate
with a (suitably protected) impersonator property to override
redirection behavior without running afoul of the chaperone invariant
and without requiring O(N) space for O(N) overrides. For example, the
contract system can implement the re-application of a contract with
different blame information by overriding blame information as
represented by properties, instead of adding a new chaperone layer
every time that blame changes.

... and all the same for non-chaperone impersonators, of course.
chaperones: allow procedure chaperones that supplies no redirection

The same as the change for structure chaperones, but for procedures.