Make EventSubscription and FilterEvents Send-able (#471)

* Make EventSubscription and FilterEvents Send-able

* Cargo fmt

* clippy

* Remove unused import

subxt/src/rpc.rs

@@ -31,15 +31,12 @@ use crate::{
     storage::StorageKeyPrefix,
     Config,
     Metadata,
+    PhantomDataSendSync,
 };
 use codec::{
     Decode,
     Encode,
 };
-use core::{
-    convert::TryInto,
-    marker::PhantomData,
-};
 use frame_metadata::RuntimeMetadataPrefixed;
 pub use jsonrpsee::{
     client_transport::ws::{
@@ -211,14 +208,14 @@ pub struct ReadProof<Hash> {
 pub struct Rpc<T: Config> {
     /// Rpc client for sending requests.
     pub client: Arc<RpcClient>,
-    marker: PhantomData<T>,
+    _marker: PhantomDataSendSync<T>,
 }
 
 impl<T: Config> Clone for Rpc<T> {
     fn clone(&self) -> Self {
         Self {
             client: self.client.clone(),
-            marker: PhantomData,
+            _marker: PhantomDataSendSync::new(),
         }
     }
 }
@@ -228,7 +225,7 @@ impl<T: Config> Rpc<T> {
     pub fn new(client: RpcClient) -> Self {
         Self {
             client: Arc::new(client),
-            marker: PhantomData,
+            _marker: PhantomDataSendSync::new(),
         }
     }