using unchecked to improve gas efficiency

[songlh]

For ERC20, _totalSupply is the sum of all values in _balances. Thus, in function _mint(), "_totalSupply += amount" does not overflow, which can guarantee all additions on _balances[account] or _balances[recipent] do not overflow.

Similarly, subtractions on _totalSupply do not overflow, because subtraction on _balances[sender] conducted earlier does not overflow.

There are several explicit checks (e.g., "senderBalance >= amount"), which can enable the use of unchecked.