graph: Add Default impls for log serializers

Author: fordN

graph/src/log/common.rs

@@ -9,6 +9,12 @@ pub struct SimpleKVSerializer {
     kvs: Vec<(String, String)>,
 }
 
+impl Default for SimpleKVSerializer {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl SimpleKVSerializer {
     pub fn new() -> Self {
         Self { kvs: Vec::new() }
@@ -39,6 +45,12 @@ pub struct VecKVSerializer {
     kvs: Vec<(String, String)>,
 }
 
+impl Default for VecKVSerializer {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl VecKVSerializer {
     pub fn new() -> Self {
         Self { kvs: Vec::new() }
@@ -61,6 +73,12 @@ pub struct HashMapKVSerializer {
     kvs: Vec<(String, String)>,
 }
 
+impl Default for HashMapKVSerializer {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl HashMapKVSerializer {
     pub fn new() -> Self {
         HashMapKVSerializer { kvs: Vec::new() }