Remove explicit S type parameters (#5275)

* Remove explicit `S` type parameters

This commit removes the explicit `S` type parameter on `Func::typed` and
`Instance::get_typed_func`. Historical versions of Rust required that
this be a type parameter but recent rustcs support a mixture of explicit
type parameters and `impl Trait`. This removes, at callsites, a
superfluous `, _` argument which otherwise never needs specification.

* Fix mdbook examples

benches/thread_eager_init.rs

@@ -55,7 +55,7 @@ fn duration_of_call(engine: &Engine, module: &Module) -> Duration {
     let mut store = Store::new(engine, ());
     let inst = Instance::new(&mut store, module, &[]).expect("instantiate");
     let f = inst.get_func(&mut store, "f").expect("get f");
-    let f = f.typed::<(), (), _>(&store).expect("type f");
+    let f = f.typed::<(), ()>(&store).expect("type f");
 
     let call = Instant::now();
     f.call(&mut store, ()).expect("call f");