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

tests/all/component_model.rs

@@ -1,10 +1,11 @@
 use anyhow::Result;
-use component_test_util::{engine, TypedFuncExt};
+use component_test_util::{async_engine, engine, TypedFuncExt};
 use std::fmt::Write;
 use std::iter;
 use wasmtime::component::Component;
 use wasmtime_component_util::REALLOC_AND_FREE;
 
+mod r#async;
 mod dynamic;
 mod func;
 mod import;