Replace mdanter/ecc with paragonie/ecc

[paragonie-security]

The PHPECC project has been abandoned for years, despite numerous attempts to fix security issues in the code. Paragon Initiative Enterprises has opted to fork PHPECC in order to provide a more secure implementation of ECDH and ECDSA for PHP projects.

Please refer to the Release Notes for our fork, which contains the specifics about the security issues we fixed and how to ensure your code is safe.