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/externref.rs

@@ -44,7 +44,7 @@ fn main() -> Result<()> {
 
     println!("Calling `externref` func...");
     let func =
-        instance.get_typed_func::<Option<ExternRef>, Option<ExternRef>, _>(&mut store, "func")?;
+        instance.get_typed_func::<Option<ExternRef>, Option<ExternRef>>(&mut store, "func")?;
     let ret = func.call(&mut store, Some(externref.clone()))?;
     assert!(ret.is_some());
     assert!(ret.unwrap().ptr_eq(&externref));