txscript: Optimize ExtractPkScriptAddrs multisig.

This continues the process of converting the ExtractPkScriptAddrs
function to use the optimized extraction functions recently introduced
as part of the typeOfScript conversion.

In particular, this converts the detection for multisig scripts.

Also, since the remaining slow path cases are all recursive calls,
the parsed opcodes are no longer used, so parsing is removed.

txscript/standard.go

@@ -860,11 +860,27 @@ func ExtractPkScriptAddrs(pkScript []byte, chainParams *chaincfg.Params) (Script
 		return PubKeyTy, addrs, 1, nil
 	}
 
+	// Check for multi-signature script.
+	const scriptVersion = 0
+	details := extractMultisigScriptDetails(scriptVersion, pkScript, true)
+	if details.valid {
+		// Convert the public keys while skipping any that are invalid.
+		addrs := make([]btcutil.Address, 0, len(details.pubKeys))
+		for _, pubkey := range details.pubKeys {
+			addr, err := btcutil.NewAddressPubKey(pubkey, chainParams)
+			if err == nil {
+				addrs = append(addrs, addr)
+			}
+		}
+		return MultiSigTy, addrs, details.requiredSigs, nil
+	}
+
 	// Fall back to slow path.  Ultimately these are intended to be replaced by
 	// faster variants based on the unparsed raw scripts.
 
 	var addrs []btcutil.Address
 	var requiredSigs int
+	var err error
 
 	// No valid addresses or required signatures if the script doesn't
 	// parse.
@@ -873,7 +889,6 @@ func ExtractPkScriptAddrs(pkScript []byte, chainParams *chaincfg.Params) (Script
 		return NonStandardTy, nil, 0, err
 	}
 
-	const scriptVersion = 0
 	scriptClass := typeOfScript(scriptVersion, pkScript)
 
 	switch scriptClass {
@@ -901,25 +916,6 @@ func ExtractPkScriptAddrs(pkScript []byte, chainParams *chaincfg.Params) (Script
 			addrs = append(addrs, addr)
 		}
 
-	case MultiSigTy:
-		// A multi-signature script is of the form:
-		//  <numsigs> <pubkey> <pubkey> <pubkey>... <numpubkeys> OP_CHECKMULTISIG
-		// Therefore the number of required signatures is the 1st item
-		// on the stack and the number of public keys is the 2nd to last
-		// item on the stack.
-		requiredSigs = asSmallInt(pops[0].opcode.value)
-		numPubKeys := asSmallInt(pops[len(pops)-2].opcode.value)
-
-		// Extract the public keys while skipping any that are invalid.
-		addrs = make([]btcutil.Address, 0, numPubKeys)
-		for i := 0; i < numPubKeys; i++ {
-			addr, err := btcutil.NewAddressPubKey(pops[i+1].data,
-				chainParams)
-			if err == nil {
-				addrs = append(addrs, addr)
-			}
-		}
-
 	case NullDataTy:
 		// Null data transactions have no addresses or required
 		// signatures.