KVStoreInterpreter::verify_signature_legacy answers true for every token and every key

Summary

`KVStoreInterpreter::verify_signature_legacy(&PushDrop, &PublicKey)`, public and deprecated since 0.2.0 (the crate's first release), read neither argument and answered `true`. A KVStore token signed by nobody, a signed token checked against a key other than its controller's, and a PushDrop with no fields each passed. Its documentation said it delegated to the `KVStoreFields`-based verifier; it did not.

An application that checks, with this function, a KVStore token it did not write (an overlay lookup's answer, or a transaction output a peer hands it) and acts on `true` accepts a value no controller signed, attributed to whatever controller the token names. No caller in the crate itself uses the function. This was run on 0.2.0, 0.5.0, 0.8.0, 0.8.7, 0.8.8, 0.9.0-beta.1 and 0.9.0-beta.7 as crates.io serves them, and the source of all 63 published versions carries the same constant body (the yanked 0.3.21 included).

0.8.9 answers `true` only for a token whose controller field is the given key and whose last field is that key's signature over the others (the 0.9 line's next prerelease carries the same fix). There is no workaround: upgrade to 0.8.9, or, on the 0.9 line, to its next prerelease.