[paymog]

jwt checked repeated member names with its own dup, which looked only at the top level of the header and claims and compared every pair of names. This switches parse (header and claims), sign, and each JWK in jwks.parse to Json.unique.parsed, which checks decoded names in every nested object at O(n log n) per object (json 0.6.1.1, #482). dup is deleted.

Behaviour change: a token whose claims repeat a name inside a nested object, such as {"cnf":{"k":1,"k":2}}, now fails with Malformed before the signature is checked, and sign of such claims fails with BadClaim. RFC 7515 §5.2 allows rejecting repeated names. A JWK with a repeated name is still skipped on its own, so the rest of the set still loads.

Checks:

• scripts/check.sh jwt passes, including the header_dup law and two new cases (verify and sign with a nested repeated name). On main, the verify case gives BadSig and the sign case succeeds.
• jwt/jwks-check.sh passes.

jwt goes to 0.7.1.0; no package imports jwt.

Closes #494
Refs #481