wasmtime: Initial, partial support for externref

This is enough to get an `externref -> externref` identity function
passing.

However, `externref`s that are dropped by compiled Wasm code are (safely)
leaked. Follow up work will leverage cranelift's stack maps to resolve this
issue.

crates/c-api/src/module.rs

@@ -16,7 +16,7 @@ wasmtime_c_api_macros::declare_ref!(wasm_module_t);
 
 impl wasm_module_t {
     fn externref(&self) -> wasmtime::ExternRef {
-        self.module.externref()
+        self.module.clone().into()
     }
 }
 
@@ -42,7 +42,7 @@ pub extern "C" fn wasmtime_module_new(
     ret: &mut *mut wasm_module_t,
 ) -> Option<Box<wasmtime_error_t>> {
     let binary = binary.as_slice();
-    let store = &store.store.borrow();
+    let store = &store.store;
     handle_result(Module::from_binary(store, binary), |module| {
         let imports = module
             .imports()
@@ -53,7 +53,7 @@ pub extern "C" fn wasmtime_module_new(
             .map(|e| wasm_exporttype_t::new(e.name().to_owned(), e.ty()))
             .collect::<Vec<_>>();
         let module = Box::new(wasm_module_t {
-            module: HostRef::new(module),
+            module: HostRef::new(store, module),
             imports,
             exports,
         });
@@ -72,7 +72,7 @@ pub extern "C" fn wasmtime_module_validate(
     binary: &wasm_byte_vec_t,
 ) -> Option<Box<wasmtime_error_t>> {
     let binary = binary.as_slice();
-    let store = &store.store.borrow();
+    let store = &store.store;
     handle_result(Module::validate(store, binary), |()| {})
 }