fix(purity): modify _submit to check both initcode and runtime

[Sabnock01]

This PR depends on #19

Overview

Contracts possess both a creationCode and a runtimeCode. At a high level, it is the job of the creationCode to load the runtimeCode into memory and then return it, in turn creating the contract. The _submit function runs the PurityChecker on the creationCode but not the runtimeCode introducing two ways to include illegal opcodes in the final runtime.

1. First, illegal opcodes could be arithmetically computed and then stored in memory using MSTORE8.
2. Second, the PurityChecker does not iterate over the bytes after pushes and instead increments the offset by the appropriate number of bytes and continues iteration. This makes it possible to push values to the stack that contain the illegal opcodes which then may be stored in memory.

It is not simply enough to check the runtimeCode and not the creationCode since not checking the latter would allow for a solver to precompute the values in the creationCode itself since in _submit the solution's deployment to _target occurs in the same execution environment as the call to run on the challenge. Since we don't want to sacrifice UX by separating these into different transactions, for a total of three for the solver to submit, we've opted to check both creationCode and runtimeCode within _submit.

Fixes

CurtaGolf.sol

• Modifies the call to the PurityChecker to check against the solution's runtime (target.code) in addition to the initcode, the _solution param.
• This check has thus been moved from before to after the deployment of _solution to target accordingly.