unsplat component::Linker::func_wrap args (#5065)

* component::Linker::func_wrap: replace IntoComponentFunc with directly accepting a closure

We find that this makes the Linker::func_wrap type signature much easier
to read. The IntoComponentFunc abstraction was adding a lot of weight to
"splat" a set of arguments from a tuple of types into individual
arguments to the closure. Additionally, making the StoreContextMut
argument optional, or the Result<return> optional, wasn't very
worthwhile.

* Fixes for the new style of closure required by component::Linker::func_wrap

* fix fuzzing generator

tests/all/component_model/strings.rs

@@ -171,12 +171,13 @@ fn test_roundtrip(engine: &Engine, src: &str, dst: &str) -> Result<()> {
     let component = Component::new(engine, &component)?;
     let mut store = Store::new(engine, String::new());
     let mut linker = Linker::new(engine);
-    linker
-        .root()
-        .func_wrap("host", |store: StoreContextMut<String>, arg: String| {
+    linker.root().func_wrap(
+        "host",
+        |store: StoreContextMut<String>, (arg,): (String,)| {
             assert_eq!(*store.data(), arg);
             Ok((arg,))
-        })?;
+        },
+    )?;
     let instance = linker.instantiate(&mut store, &component)?;
     let func = instance.get_typed_func::<(String,), (String,), _>(&mut store, "echo")?;