Signature::from_der reads a signature with bytes after it, a wrong sequence length or an unneeded zero as the signature

Summary

`Signature::from_der` read a DER-encoded ECDSA signature leniently. It read as the signature: the signature followed by other bytes; a sequence length shorter than its contents, or longer with bytes after them (a length byte with its high bit set among them); and an R or S preceded by a zero byte it did not need. It read a lone-zero R or S as zero.

`messages::verify`, `compat::bsm::verify_message_der`, `auth::Certificate::verify`, `auth::VerifiableCertificate::verify`, `wallet::ProtoWallet::verify_signature` and `kvstore::KVStoreInterpreter::verify_signature` read through it, and each verified such a form of a valid signature. Given one signature a key holder made, anyone could produce other byte strings that verify as that signature over the same data. No form verifies over other data: an application that acts on the verdict alone is not affected; one that compares, hashes or stores a signature's bytes is.

0.8.6 refuses each form as `Error::InvalidSignature`. A signature's bytes are still not unique after the patch: the same signature with S replaced by n - S verifies, so an application that keys on the bytes also requires `Signature::is_low_s()`.