Remove SafeMath for 0.8.0

[alxiong]

based on OpenZeppelin's doc and solidity's 0.8.0 release, Solidity already come with checked arithmetics after 0.8.0.

This issue try to remove it from our code (but don't remove openzeppelin dependencies, since we still need it for ERC token standards, and just would use for most project).
We also need to update the writeup.