Change EC backend

[ofek]

Consider switching from the current OpenSSL hack/pure Python implementation to https://github.com/ofek/coincurve.

Features:

• Clean 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
• Implements a fix for https://bugs.python.org/issue28150 to support Python 3.6+ on macOS

See:

raiden-network/raiden#534
ethereum/pyethereum#713