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/fuel.rs

@@ -15,7 +15,7 @@ fn main() -> Result<()> {
     let instance = Instance::new(&mut store, &module, &[])?;
 
     // Invoke `fibonacci` export with higher and higher numbers until we exhaust our fuel.
-    let fibonacci = instance.get_typed_func::<i32, i32, _>(&mut store, "fibonacci")?;
+    let fibonacci = instance.get_typed_func::<i32, i32>(&mut store, "fibonacci")?;
     for n in 1.. {
         let fuel_before = store.fuel_consumed().unwrap();
         let output = match fibonacci.call(&mut store, n) {