Deny unused crate dependencies (#549)

* subxt unused crate deps

* Deny unused dependencies

subxt/src/lib.rs

@@ -35,6 +35,7 @@
     while_true,
     trivial_casts,
     trivial_numeric_casts,
+    unused_crate_dependencies,
     unused_extern_crates,
     clippy::all
 )]