GAS opcode should be included in purity checker

Issue

Purity checker does not currently include the GAS opcode. This opcode can reveal information about the outside world and can make the result of the function not entirely deterministic upon its input arguments.

Proposed Implementation

• Add GAS opcode (0x5a) to the purity checker
• Compile to bytecode and integrate into testing setup in conftest.py
• Update deploy scripts

[ralexstokes]

@gitcoinbot refer #169

[djrtwo]

Purity of contracts defined by this purity checker need to give a deterministic output only on the parameters pass in, not on the state of the blockchain at the time. An example could be something like the following:

def true_if_even_gas():
    return GAS % 2 == 0

This function is not deterministic on its inputs but rather on the state of the context of its execution. If a validator’s signature had such a triviality, they could sign a message that would validate as True in one context but not in another. Thus opening up the potential of being able to sign nefarious messages and not get slashed for it.