component model: async host function & embedding support (#5055)

* func_wrap_async typechecks

* func call async

* instantiate_async

* fixes

* async engine creation for tests

* start adding a component model test for async

* fix wrong check for async support, factor out Instance::new_started to an unchecked impl

* tests: wibbles

* 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

* future of result of return

* add Linker::instantiate_async and {Typed}Func::post_return_async

* fix fuzzing generator

* note optimisation opportunity

* simplify test

crates/misc/component-test-util/src/lib.rs

@@ -65,6 +65,12 @@ pub fn engine() -> Engine {
     Engine::new(&config()).unwrap()
 }
 
+pub fn async_engine() -> Engine {
+    let mut config = config();
+    config.async_support(true);
+    Engine::new(&config).unwrap()
+}
+
 /// Newtype wrapper for `f32` whose `PartialEq` impl considers NaNs equal to each other.
 #[derive(Copy, Clone, Debug, Arbitrary)]
 pub struct Float32(pub f32);