Elem fields should be Option

E.g. for the set:
set test_set {
    typeof ip saddr
    flags interval
    elements = { 10.0.0.0/24 }
}

The json object will look like this:
 "set": {
    "family": "ip",
    "name": "snat_hv",
    "table": "nat",
    "type": "ipv4_addr",
    "handle": 4,
    "flags": [
      "interval"
    ],
    "elem": [
      {
        "elem": {
          "val": {
            "prefix": {
              "addr": "10.0.0.0",
              "len": 24
            }
          },
        }
      }
 }
 There is no timeout, expires and comments fields, they should be
 optional.

src/expr.rs

@@ -351,14 +351,21 @@ pub enum Verdict {
     Goto(String),
 }
 
+#[derive(Debug, Clone, Eq, PartialEq, Serialize, Deserialize)]
+/// Explicitly set element object.
+pub struct ElemCounter {
+    pub packets: u32,
+    pub bytes: u32,
+}
+
 #[derive(Debug, Clone, Eq, PartialEq, Serialize, Deserialize)]
 #[serde(rename = "elem")]
 /// Explicitly set element object.
 pub struct Elem {
     pub val: Box<Expression>,
-    pub timeout: u32,
-    pub expires: u32,
-    pub comment: String,
+    pub timeout: Option<u32>,
+    pub expires: Option<u32>,
+    pub comment: Option<String>,
     pub counter: Option<Counter>,
 }