Guard opaque predicates with an (-> Any Any) contract. This uses the
contract generation infrastructure to avoid wrapping struct predicates.
Also, relax `any-wrap/c` (the contract used for `Any` in positive
position) to allow opaque structures. This also requires an enumeration
of all the other kinds of values that TR understands, so that they are
not confused with opaque structures.
Joint work with @bennn.
Closes#202.
Closes#203.
Closes#241.
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