Zcash Optimisations and Montgomery Curve

[HarryR]

This implements some of the optimisations from ZCash for faster Pedersen hashing, using a signed 3bit window.

It adds the following gadgets:

• lookup_signed_3bit_gadget
• fixed_base_mul_zcash
• montgomery::PointAdder
• montgomery::EdwardConversion

It also adds a Point class with methods for addition and conversion.

There is some overlap between https://github.com/HarryR/ethsnarks/pull/80 and this pull request, and there are some cleanups I'd like to make to this. I'm figuring how how to merge the two.

[HarryR]

This has been merged into https://github.com/HarryR/ethsnarks/pull/80