Use Yul optimizer

[pawurb]

This PR enables optimizer for Yul and also fixes issue from #2 .

I'm seeing ~40% bytecode size reduction with optimization enabled on my test project https://github.com/pawurb/yul721

solc --strict-assembly src/Yul721.yul --bin | tail -1 | wc // 4697
solc --strict-assembly src/Yul721.yul --bin --optimize | tail -1 | wc // 2941

and also ~0.5% smaller gas usage.