Eddsa cleanup

[HarryR]

• Split merkle_path into header and implementation
• Cleanup ethsnarks.eddsa module
• Improve documentation for EdDSA
• Added base point as Point.generator()
• Easier to use different algorithms with ethsnarks.eddsa
• Use bitstring.BitArray as the native type for bitwise data
• fixed bug in subadd gadget with large integer sizes
• removed node-ffi and ref dependencies from NPM packages.json (speeds up build time)

Required for the ethsnarks-snasma example project