stackitem: avoid going through Value() in serialization

Doesn't change much, but still simpler.

name               old time/op    new time/op    delta
SerializeSimple-8     452ns ±10%     435ns ± 4%   ~     (p=0.356 n=10+9)

name               old alloc/op   new alloc/op   delta
SerializeSimple-8      432B ± 0%      432B ± 0%   ~     (all equal)

name               old allocs/op  new allocs/op  delta
SerializeSimple-8      7.00 ± 0%      7.00 ± 0%   ~     (all equal)

pkg/vm/stackitem/serialization_test.go

@@ -207,3 +207,15 @@ func BenchmarkEncodeBinary(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkSerializeSimple(b *testing.B) {
+	s := NewStruct(nil)
+	s.Append(Make(100500))
+	s.Append(Make("1aada0032aba1ef6d1f0")) // Mimicking uint160.
+	for i := 0; i < b.N; i++ {
+		_, err := Serialize(s)
+		if err != nil {
+			b.FailNow()
+		}
+	}
+}