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_test.go

@@ -226,22 +226,36 @@ func TestSwapElemValues(t *testing.T) {
 	s.PushVal(2)
 	s.PushVal(4)
 
-	a := s.Peek(0)
-	b := s.Peek(1)
-
-	// [ 4 ] -> a
-	// [ 2 ] -> b
-
-	aval := a.value
-	bval := b.value
-	a.value = bval
-	b.value = aval
-
-	// [ 2 ] -> a
-	// [ 4 ] -> b
-
+	assert.NoError(t, s.Swap(0, 1))
 	assert.Equal(t, int64(2), s.Pop().BigInt().Int64())
 	assert.Equal(t, int64(4), s.Pop().BigInt().Int64())
+
+	s.PushVal(1)
+	s.PushVal(2)
+	s.PushVal(3)
+	s.PushVal(4)
+
+	assert.NoError(t, s.Swap(1, 3))
+	assert.Equal(t, int64(4), s.Pop().BigInt().Int64())
+	assert.Equal(t, int64(1), s.Pop().BigInt().Int64())
+	assert.Equal(t, int64(2), s.Pop().BigInt().Int64())
+	assert.Equal(t, int64(3), s.Pop().BigInt().Int64())
+
+	s.PushVal(1)
+	s.PushVal(2)
+	s.PushVal(3)
+	s.PushVal(4)
+
+	assert.Error(t, s.Swap(-1, 0))
+	assert.Error(t, s.Swap(0, -3))
+	assert.Error(t, s.Swap(0, 4))
+	assert.Error(t, s.Swap(5, 0))
+
+	assert.NoError(t, s.Swap(1, 1))
+	assert.Equal(t, int64(4), s.Pop().BigInt().Int64())
+	assert.Equal(t, int64(3), s.Pop().BigInt().Int64())
+	assert.Equal(t, int64(2), s.Pop().BigInt().Int64())
+	assert.Equal(t, int64(1), s.Pop().BigInt().Int64())
 }
 
 func TestPopSigElements(t *testing.T) {