Implement the ZkProgram

[mitschabaude]

After #7 is merged, the function that creates a zkprogram from a given spec is stubbed out. Now we want to implement it as much of it as possible!

• For attestation, call the verify() function
• For operations, do the corresponding circuit operations
• For an assert return, assert the boolean input to be true
• For a data return, return it as public output