feat: Diffie-Hellman and elliptic curve DH

[lonerapier]

Since we have ECDSA now, it would be fascinating to see a toy implementation of Diffie-Hellman and ECDH.

Resources:

• https://web.math.ucsb.edu/~kylehansen/Papers/Elliptic_Curve_Cryptography.pdf
• http://koclab.cs.ucsb.edu/teaching/ecc/project/2015Projects/Haakegaard+Lang.pdf
• https://www.gabriel.urdhr.fr/2021/10/19/diffie-hellman-intro/