Optimize gas estimation

From my tests, it reduces the number of tx simulations for each gas estimation down from 16-18 to just 2 for most transactions (actually for all the transactions that I've tried).

core/state_transition.go

@@ -35,6 +35,7 @@ type ExecutionResult struct {
 	UsedGas    uint64 // Total used gas but include the refunded gas
 	Err        error  // Any error encountered during the execution(listed in core/vm/errors.go)
 	ReturnData []byte // Returned data from evm(function result or data supplied with revert opcode)
+	GasRefund  uint64 // Total gas refunded
 }
 
 // Unwrap returns the internal evm error which allows us for further
@@ -445,6 +446,7 @@ func (st *StateTransition) TransitionDb() (*ExecutionResult, error) {
 		UsedGas:    st.gasUsed(),
 		Err:        vmerr,
 		ReturnData: ret,
+		GasRefund:	st.state.GetRefund(),
 	}, nil
 }
 

internal/ethapi/api.go

@@ -1279,6 +1279,14 @@ func DoEstimateGas(ctx context.Context, b Backend, args TransactionArgs, blockNr
 		}
 		return 0, fmt.Errorf("gas required exceeds allowance (%d)", hi)
 	}
+
+	// Optimization: if the transaction succeeded with gasLimit set to the first execution's 
+	// usedGas + gasRefund, then return that value immediately. Else, continue with the binary search.
+	_, _, err = executeEstimate(ctx, b, args, state.Copy(), header, gasCap, result.UsedGas + result.GasRefund)
+	if err == nil {
+		return hexutil.Uint64(result.UsedGas + result.GasRefund), nil
+	}
+
 	// For almost any transaction, the gas consumed by the unconstrained execution above
 	// lower-bounds the gas limit required for it to succeed. One exception is those txs that
 	// explicitly check gas remaining in order to successfully execute within a given limit, but we