VK summary

[imeckler]

This PR adds verification key 'summaries' to the snarky verifiers.

Basically we construct a (non-deterministic, non-total) function summary : G1.t list * G2.t list * Fqk.Unitary.t list -> bool list which is injective (if it doesn't throw). By Fqk.Unitary.t I mean Fqk elements which are unitary. a + b beta : Fqk.t is unitary if (a + b beta)(a - b beta) = 1.

The summary works element-wise as follows:

• G1 elements are mapped to their x coordinate and the parity of their y coordinate
• G2 elements are mapped to their x coordinate and the parity of the 'real part' of their y coordinate, assuming the real part is non-zero
• Fqk.t elements a + b beta are mapped to a and the parity of the 'real part' of b, assuming the real part is non-zero. b is determined up to sign by a by unitary-ness.

The point of this is for making hashing more efficient. summary shrinks the size of its input, so you can get away with hashing fewer bits.

[bkase]

Should we be consistent with style and do an inner module Checked here instead of a suffix _unchecked?