Add ECC encryption and decryption tool method to core

[doubiliu]

Summary or problem description
The key distribution process of the random number on the chain requires asymmetric encryption of the key(neo-project/neo-modules#637 (comment)). At the same time, a data encryption function can be added to the Fs client. So we can add an asymmetric encryption and decryption tool method to the core Cryptography, easy to use.

Do you have any solution you want to propose?

Process

Encrypt

• Generate a symmetric encryption private key
• Use the symmetric encryption private key to encrypt data
• Use the recipient's public key to encrypt the symmetric encryption private key
• Send the encrypted symmetric encryption private key and data to the receiver

Decrypt

• The receiver receives the data and obtains the encrypted symmetric encryption private key and data from it
• Use your own private key to decrypt the encrypted symmetric encryption private key
• Use symmetric encryption private key to decrypt encrypted data

Neo Version

• Neo 3

Where in the software does this update applies to?

• Cryptography

If you think it is possible, I can complete the relevant PR.

[doubiliu]

@erikzhang