Switch secp256k1 dependency

[ofek]

@ulope told me the Ethereum sphere has interest in supporting Windows.

Consider switching from https://github.com/ludbb/secp256k1-py to https://github.com/ofek/coincurve as the former is no longer maintained.

New features include:

• Cleaner API
• Uses newest version of libsecp256k1
• Support for Windows
• Linux, macOS, and Windows all have binary packages for both 64 and 32-bit architectures
• Linux & macOS use GMP for faster computation
• Endomorphism optimization is enabled
• A global context is used by default, drastically increasing performance
• Fixed ECDH
• A fix to remove CFFI warnings
• Implements a fix for https://bugs.python.org/issue28150 to support Python 3.6+ on macOS

[ofek]

Added in #777