txscript: Refactor to implement tokenizer from drcd

parseScriptTemplate will allocate a copy of the script onto memory,
leading to excessive memory allocation. This commit brings the tokenizer
to btcd and modifies it to fit with Bitcoin.

blockchain/validate.go

@@ -403,8 +403,7 @@ func CountP2SHSigOps(tx *btcutil.Tx, isCoinBaseTx bool, utxoView *UtxoViewpoint)
 		// Count the precise number of signature operations in the
 		// referenced public key script.
 		sigScript := txIn.SignatureScript
-		numSigOps := txscript.GetPreciseSigOpCount(sigScript, pkScript,
-			true)
+		numSigOps := txscript.GetPreciseSigOpCount(sigScript, pkScript)
 
 		// We could potentially overflow the accumulator so check for
 		// overflow.

mempool/policy.go

@@ -103,7 +103,7 @@ func checkInputsStandard(tx *btcutil.Tx, utxoView *blockchain.UtxoViewpoint) err
 		switch txscript.GetScriptClass(originPkScript) {
 		case txscript.ScriptHashTy:
 			numSigOps := txscript.GetPreciseSigOpCount(
-				txIn.SignatureScript, originPkScript, true)
+				txIn.SignatureScript, originPkScript)
 			if numSigOps > maxStandardP2SHSigOps {
 				str := fmt.Sprintf("transaction input #%d has "+
 					"%d signature operations which is more "+