serialize partial_fee into string (#4898)

* serialize partial_fee into string

* implement deserialize

* bump version

substrate/bin/node/runtime/src/lib.rs

@@ -82,7 +82,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
 	spec_version: 216,
-	impl_version: 1,
+	impl_version: 2,
 	apis: RUNTIME_API_VERSIONS,
 };