vm: optimize SWAP instruction, refactor common code

Add `Swap` method to the Stack and use it for both SWAP and XSWAP. Avoid
element popping and pushing (and associated accounting costs).

1.4M->1.5M 100K block import test before:
real    3m51,885s
user    5m54,744s
sys     0m38,444s

After:
real    3m44,292s
user    5m43,494s
sys     0m34,741s

pkg/vm/stack.go

@@ -2,6 +2,7 @@ package vm
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"math/big"
 
@@ -371,6 +372,23 @@ func (s *Stack) IterBack(f func(*Element)) {
 	}
 }
 
+// Swap swaps two elements on the stack without popping and pushing them.
+func (s *Stack) Swap(n1, n2 int) error {
+	if n1 < 0 || n2 < 0 {
+		return errors.New("negative index")
+	}
+	if n1 >= s.len || n2 >= s.len {
+		return errors.New("too big index")
+	}
+	if n1 == n2 {
+		return nil
+	}
+	a := s.Peek(n1)
+	b := s.Peek(n2)
+	a.value, b.value = b.value, a.value
+	return nil
+}
+
 // popSigElements pops keys or signatures from the stack as needed for
 // CHECKMULTISIG.
 func (s *Stack) popSigElements() ([][]byte, error) {