Commit 5bae9773aa broke `chaperone-procedure` and `impersonate-procedure`
so that it didn't always produce a chaperone or impersonator. Also,
the arity-error changeds intended for that commit were not complete,
because tests were accidentally commented out.
The main repair involves a new internal property that keeps track of an
accessor that is used to extract a procedure from a field in a structure
type that has `prop:procedure`.
4070bcd461