Support ephemeral private key for generating ecdh shared key for encryption

[Chengxuan]

resolves #47

• update circuits with encryption to accept an additional input (ecdh private key)
• ECDH shared key is now generated using the additional private key (user is encouraged to use random ephemeral keys to reduce the blast radius of losing private keys) and the receiver's public key
• the public key of the ephemeral private key used for generating the ecdh shared key is published in the transfer events so the receiver can generate it using its private key.

in PR chain with #83

[jimthematrix]

looks great to me!