gas ops for for-loops

[RonTuretzky]

• ++i is cheaper than i++
• after sol 0.8.0, can use unchecked to increment vars that have no risk of overflow
• cache the length in a var

example of most efficient solidity for-loop:

uint256 len = arr.length;
for (uint256 i; i < len) {
   // logic before increment or it won't start at zero
   unchecked { ++i }
}