Extend `define-cstruct' to support #:property specs, which causes
the constructor and C->Racket coercsions to wrap the pointer in
a structure instance with the specified properties. Of course,
the wrapper structure has a `prop:cpointer' property so that the
wrapper can be used transparently as a C pointer.
Add missing tests and documentation for the id`->list', `list->'id,
id`->list*', and `list*->'id bindings created by `define-cstruct'.
