Add the ability to cache typechecking an instance (#2962)

* Add the ability to cache typechecking an instance

This commit adds the abilty to cache the type-checked imports of an
instance if an instance is going to be instantiated multiple times. This
can also be useful to do a "dry run" of instantiation where no wasm code
is run but it's double-checked that a `Linker` possesses everything
necessary to instantiate the provided module.

This should ideally help cut down repeated instantiation costs slightly
by avoiding type-checking and allocation a `Vec<Extern>` on each
instantiation. It's expected though that the impact on instantiation
time is quite small and likely not super significant. The functionality,
though, of pre-checking can be useful for some embeddings.

* Fix build with async

crates/wasmtime/src/func.rs

@@ -677,7 +677,7 @@ impl Func {
     /// initiates a panic. Also panics if `store` does not own this function.
     pub fn call(&self, mut store: impl AsContextMut, params: &[Val]) -> Result<Box<[Val]>> {
         assert!(
-            !cfg!(feature = "async") || !store.as_context().async_support(),
+            !store.as_context().async_support(),
             "must use `call_async` when async support is enabled on the config",
         );
         let my_ty = self.ty(&store);
@@ -1894,6 +1894,10 @@ impl HostFunc {
         let idx = self.export.anyfunc.as_ref().type_index;
         store.register_host_trampoline(idx, self.trampoline);
     }
+
+    pub(crate) fn sig_index(&self) -> VMSharedSignatureIndex {
+        unsafe { self.export.anyfunc.as_ref().type_index }
+    }
 }
 
 impl Drop for HostFunc {

crates/wasmtime/src/func/typed.rs

@@ -73,7 +73,7 @@ where
     pub fn call(&self, mut store: impl AsContextMut, params: Params) -> Result<Results, Trap> {
         let mut store = store.as_context_mut().opaque();
         assert!(
-            !cfg!(feature = "async") || !store.async_support(),
+            !store.async_support(),
             "must use `call_async` with async stores"
         );
         unsafe { self._call(&mut store, params) }