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

crates/test-programs/tests/wasm_tests/runtime/cap_std_sync.rs

@@ -59,7 +59,7 @@ fn run(
 
         let mut store = Store::new(&engine, builder.build());
         let instance = linker.instantiate(&mut store, &module)?;
-        let start = instance.get_typed_func::<(), (), _>(&mut store, "_start")?;
+        let start = instance.get_typed_func::<(), ()>(&mut store, "_start")?;
         start.call(&mut store, ()).map_err(anyhow::Error::from)
     };
 

crates/test-programs/tests/wasm_tests/runtime/tokio.rs

@@ -67,7 +67,7 @@ fn run(
             let mut store = Store::new(&engine, builder.build());
 
             let instance = linker.instantiate_async(&mut store, &module).await?;
-            let start = instance.get_typed_func::<(), (), _>(&mut store, "_start")?;
+            let start = instance.get_typed_func::<(), ()>(&mut store, "_start")?;
             start
                 .call_async(&mut store, ())
                 .await