Stepping back a little, a chaperone constructor for a datatype often
needs some inside information and cooperation from the dataype
implementation. For example, `chaperone-hash` has the property that you
want --- the filter applied to a `hash-ref` result doesn't get used if
a failure thunk produces the `hash-ref` result --- and that works
because `hash-ref` and `chaperone-hash` are implemented together.
I'm guessing that you don't have access to a dictionary implementation
because new kinds of dictionaries can be created by implementing
`gen:dict`. Maybe `dict/c` based on chaperones can only work for
dictionaries that supply additional implementation hooks by also
implementing `gen:dict-chaperone` (or something like that).
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