Removed rmp-serde dependency

Also flattened the client message struct because theres no reason to
nest a 2-element map at the top level

src/client.rs

@@ -23,7 +23,7 @@ pub struct TaggedClientMessage {
 pub struct SignedClientMessage {
     message: ClientMessage,
     // Should I enforce this being a ecdsa signature?
-    #[serde(with = "serde_bytes")]
+    #[serde(with = "serde_bytes",flatten)]
     signature: Box<[u8]>
 }