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/threads.rs

@@ -52,7 +52,7 @@ fn run(engine: &Engine, module: &Module, linker: &Linker<()>) -> Result<()> {
     println!("Instantiating module...");
     let mut store = Store::new(&engine, ());
     let instance = linker.instantiate(&mut store, module)?;
-    let run = instance.get_typed_func::<(), (), _>(&mut store, "run")?;
+    let run = instance.get_typed_func::<(), ()>(&mut store, "run")?;
 
     println!("Executing...");
     for _ in 0..N_REPS {