rpcserver: Optimize JSON raw tx input list create.

This optimizes the createVinList function which is used to generate the
JSON list of transaction inputs.  It also makes it more consistent with
the createVinListPrevOut function.

In particular, it entails the following changes:
- Only do a single coinbase check and return right away instead of
  checking multiple times inside the loop over the inputs
- Use a pointer for populating the details of each entry to avoid
  multiple unnecessary array lookups and bounds checks
- Group all fields that populate the entry for better readability

rpcserver.go

@@ -1296,27 +1296,37 @@ func handleDebugLevel(s *rpcServer, cmd interface{}, closeChan <-chan struct{})
 // createVinList returns a slice of JSON objects for the inputs of the passed
 // transaction.
 func createVinList(mtx *wire.MsgTx) []dcrjson.Vin {
+	// Coinbase transactions only have a single txin by definition.
 	vinList := make([]dcrjson.Vin, len(mtx.TxIn))
-	for i, v := range mtx.TxIn {
-		if blockchain.IsCoinBaseTx(mtx) {
-			vinList[i].Coinbase = hex.EncodeToString(v.SignatureScript)
-		} else {
-			vinList[i].Txid = v.PreviousOutPoint.Hash.String()
-			vinList[i].Vout = v.PreviousOutPoint.Index
-			vinList[i].Tree = v.PreviousOutPoint.Tree
+	if blockchain.IsCoinBaseTx(mtx) {
+		txIn := mtx.TxIn[0]
+		vinEntry := &vinList[0]
+		vinEntry.Coinbase = hex.EncodeToString(txIn.SignatureScript)
+		vinEntry.Sequence = txIn.Sequence
+		vinEntry.AmountIn = txIn.ValueIn
+		vinEntry.BlockHeight = txIn.BlockHeight
+		vinEntry.BlockIndex = txIn.BlockIndex
+		return vinList
+	}
 
-			// The disassembled string will contain [error] inline
-			// if the script doesn't fully parse, so ignore the
-			// error here.
-			disbuf, _ := txscript.DisasmString(v.SignatureScript)
-			vinList[i].ScriptSig = new(dcrjson.ScriptSig)
-			vinList[i].ScriptSig.Asm = disbuf
-			vinList[i].ScriptSig.Hex = hex.EncodeToString(v.SignatureScript)
+	for i, txIn := range mtx.TxIn {
+		// The disassembled string will contain [error] inline
+		// if the script doesn't fully parse, so ignore the
+		// error here.
+		disbuf, _ := txscript.DisasmString(txIn.SignatureScript)
+
+		vinEntry := &vinList[i]
+		vinEntry.Txid = txIn.PreviousOutPoint.Hash.String()
+		vinEntry.Vout = txIn.PreviousOutPoint.Index
+		vinEntry.Tree = txIn.PreviousOutPoint.Tree
+		vinEntry.Sequence = txIn.Sequence
+		vinEntry.AmountIn = txIn.ValueIn
+		vinEntry.BlockHeight = txIn.BlockHeight
+		vinEntry.BlockIndex = txIn.BlockIndex
+		vinEntry.ScriptSig = &dcrjson.ScriptSig{
+			Asm: disbuf,
+			Hex: hex.EncodeToString(txIn.SignatureScript),
 		}
-		vinList[i].AmountIn = v.ValueIn
-		vinList[i].BlockHeight = v.BlockHeight
-		vinList[i].BlockIndex = v.BlockIndex
-		vinList[i].Sequence = v.Sequence
 	}
 
 	return vinList
@@ -1325,7 +1335,6 @@ func createVinList(mtx *wire.MsgTx) []dcrjson.Vin {
 // createVinList returns a slice of JSON objects for the inputs of the passed
 // transaction.
 func createVinListPrevOut(s *rpcServer, mtx *wire.MsgTx, chainParams *chaincfg.Params, vinExtra int) []dcrjson.VinPrevOut {
-
 	// Coinbase transactions only have a single txin by definition.
 	vinList := make([]dcrjson.VinPrevOut, len(mtx.TxIn))
 	if blockchain.IsCoinBaseTx(mtx) {