Revert "Remove the need for HostRef<Module> (#778)"

This reverts commit 7b33f1c619.

Pushed a few extra commits by accident, so reverting this.

crates/api/tests/import_calling_export.rs

@@ -34,7 +34,7 @@ fn test_import_calling_export() {
 
     let store = Store::default();
     let wasm = wat::parse_str(WAT).unwrap();
-    let module = Module::new(&store, &wasm).expect("failed to create module");
+    let module = HostRef::new(Module::new(&store, &wasm).expect("failed to create module"));
 
     let callback = Rc::new(Callback {
         other: RefCell::new(None),

crates/api/tests/import_calling_export.wat

@@ -0,0 +1,8 @@
+(module
+  (type $t0 (func))
+  (import "" "imp" (func $.imp (type $t0)))
+  (func $run call $.imp)
+  (func $other)
+  (export "run" (func $run))
+  (export "other" (func $other))
+)

crates/api/tests/traps.rs

@@ -15,16 +15,17 @@ fn test_trap_return() -> Result<(), String> {
     let store = Store::default();
     let binary = parse_str(
         r#"
-            (module
-            (func $hello (import "" "hello"))
-            (func (export "run") (call $hello))
-            )
-        "#,
+                (module
+                (func $hello (import "" "hello"))
+                (func (export "run") (call $hello))
+                )
+            "#,
     )
     .map_err(|e| format!("failed to parse WebAssembly text source: {}", e))?;
 
-    let module =
-        Module::new(&store, &binary).map_err(|e| format!("failed to compile module: {}", e))?;
+    let module = HostRef::new(
+        Module::new(&store, &binary).map_err(|e| format!("failed to compile module: {}", e))?,
+    );
     let hello_type = FuncType::new(Box::new([]), Box::new([]));
     let hello_func = HostRef::new(Func::new(&store, hello_type, Rc::new(HelloCallback)));