Update Solidity version, eliminate external dependency on safeMath, and refactor arithmetic

[floor-licker]

I'd like to do this for the entire codebase. All .sol files would have their version updated to ^0.8.0 so that the external dependency on the safeMath library is eliminated, and the contracts will instead use the more robust and efficient built-in arithmetic of the 0.8.0 compiler. The main reason for doing this is gas efficiency, but also the only reason for using safeMath in older versions was for protection against overflow/underflow, but these checks are built-in to the Solidity compiler for versions >=0.8.0. If this update is approved I can submit the PR immediately.