Use serde_json to export hardcoded sync

The exported hardcoded sync was previously generating invalid JSON, with
the CHT list ending in a trailing comma. In order to remedy this, this
commit uses serde_json to serialize the SpecHardcodedSync struct
into valid JSON.

Fixes #11415

ethcore/spec/Cargo.toml

@@ -32,6 +32,8 @@ maplit = "1"
 null-engine = { path = "../engines/null-engine" }
 pod = { path = "../pod" }
 rlp = "0.4.2"
+serde = "1.0"
+serde_json = "1.0"
 trace = { path = "../trace" }
 trie-vm-factories = { path = "../trie-vm-factories" }
 vm = { path = "../vm" }

ethcore/spec/src/spec.rs

@@ -51,6 +51,8 @@ use maplit::btreeset;
 use null_engine::NullEngine;
 use pod::PodState;
 use rlp::{Rlp, RlpStream};
+use serde::ser::{Serialize, Serializer, SerializeStruct};
+use serde_json;
 use trace::{NoopTracer, NoopVMTracer};
 use trie_vm_factories::Factories;
 use vm::{EnvInfo, ActionType, ActionValue, ActionParams, ParamsType};
@@ -264,15 +266,29 @@ impl From<ethjson::spec::HardcodedSync> for SpecHardcodedSync {
 	}
 }
 
+impl Serialize for SpecHardcodedSync {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+        where
+            S: Serializer,
+        {
+            let mut serialized = serializer.serialize_struct("SpecHardcodedSync", 3)?;
+
+            let header_str = format!("{:x}", self.header);
+            serialized.serialize_field("header", &header_str)?;
+
+            let total_difficulty_str = format!("{:?}", self.total_difficulty);
+            serialized.serialize_field("totalDifficulty", &total_difficulty_str)?;
+
+            serialized.serialize_field("CHTs", &self.chts)?;
+            serialized.end()
+        }
+}
+
 impl fmt::Display for SpecHardcodedSync {
-	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-		writeln!(f, "{{")?;
-		writeln!(f, r#""header": "{:x}","#, self.header)?;
-		writeln!(f, r#""totalDifficulty": "{:?}""#, self.total_difficulty)?;
-		// TODO: #11415 - fix trailing comma for CHTs
-		writeln!(f, r#""CHTs": {:#?}"#, self.chts.iter().map(|x| format!("{:?}", x)).collect::<Vec<_>>())?;
-		writeln!(f, "}}")
-	}
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let serialized = serde_json::to_string_pretty(&self).unwrap();
+        writeln!(f, "{}", serialized)
+    }
 }
 
 fn convert_json_to_spec(