STARK for Rescue hash function

[bobbinth]

This will address #1 . Seems like Rescue is simpler to implement - so, that's what we'll do.

The plan is:

• [x] Make defining complex transition functions and constraints simpler.
  • [x] Implement basic arithmetic script to enable intermediate variables
  • [x] Add support for vector/matrix operations
• [x] Create STARK for simple Rescue example: 2 registers, 64-bits field, 32 rounds (similar to the one in the white paper).
• [x] Create a more "real-life" STARK. Maybe; 4 registers, 128-bit field, 32 rounds. This should have about 255 bits of security.
• [x] Update documentation.
• [x] Clean up code and do more testing.