Gas Optimizations

[code423n4]

• Repeated operations should be cached, e.g. amount.toInt256() is calculated twice:

  require(margin[idx][trader] >= amount.toInt256(), "Insufficient balance");
  margin[idx][trader] -= amount.toInt256();

• ++ is a bit cheaper:

  i += 1;

• 'withdrawals' and 'start' state variables accessed multiple times in a loop should be cached before the loop:

  while (i < withdrawals.length && (i - start) <= maxWithdrawalProcesses)

[atvanguard]

Duplicate of #21