Replace prover's identity in circuit from decryption_key to proof_generation_key

[osuketh]

Currently, the decryption_key is used for Encryption proving in circuit. It's not problematic as long as you won't delegate zkproofs or calling transfer_from function (#133).

For supporting these features, proof_generation_key should be passed to circuit, but it will cause to increase constraints about 25,000 because computation of collision resistance hash (blake2s) will be needed to derive decryption_keyfrom proof_generation_key.