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

tests/all/import_indexes.rs

@@ -43,7 +43,7 @@ fn same_import_names_still_distinct() -> anyhow::Result<()> {
     ];
     let instance = Instance::new(&mut store, &module, &imports)?;
 
-    let func = instance.get_typed_func::<(), i32, _>(&mut store, "foo")?;
+    let func = instance.get_typed_func::<(), i32>(&mut store, "foo")?;
     let result = func.call(&mut store, ())?;
     assert_eq!(result, 3);
     Ok(())