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

examples/tokio/main.rs

@@ -106,7 +106,7 @@ async fn run_wasm(inputs: Inputs) -> Result<(), Error> {
         .instantiate_async(&mut store, &inputs.env.module)
         .await?;
     instance
-        .get_typed_func::<(), (), _>(&mut store, "_start")?
+        .get_typed_func::<(), ()>(&mut store, "_start")?
         .call_async(&mut store, ())
         .await?;