Reduce event information

[duncancmt]

0x-settler/src/core/OtcOrderSettlement.sol

Lines 80 to 83 in d258b1b

	// TODO actually calculate the orderHash
	emit OtcOrderFilled(
	witness, order.maker, taker, order.makerToken, order.takerToken, order.makerAmount, order.takerAmount
	);

In keeping with our philosophy of reducing the gas consumed by emitting events, can this event be pruned down to contain just the witness? The witness uniquely identifies the order itself, and the actual fill amounts can be inferred from the Transfer events fired by the 2 tokens.