Refactor externref related APIs of reference types feature (#971)

Currently when calling wasm_runtime_call_wasm() to invoke wasm function
with externref type argument from runtime embedder, developer needs to
use wasm_externref_obj2ref() to convert externref obj into an internal ref
index firstly, which is not convenient to developer.
To align with GC feature in which all the references passed to
wasm_runtime_call_wasm() can be object pointers directly, we change the
interface of wasm_runtime_call_wasm() to allow to pass object pointer
directly for the externref argument, and refactor the related codes, update
the related samples and the document.

core/iwasm/aot/aot_runtime.c

@@ -85,7 +85,7 @@ init_global_data(uint8 *global_data, uint8 type, WASMValue *initial_value)
     switch (type) {
         case VALUE_TYPE_I32:
         case VALUE_TYPE_F32:
-#if WASM_ENABLE_REF_TYPES
+#if WASM_ENABLE_REF_TYPES != 0
         case VALUE_TYPE_FUNCREF:
         case VALUE_TYPE_EXTERNREF:
 #endif
@@ -1572,16 +1572,8 @@ aot_create_exec_env_and_call_function(AOTModuleInstance *module_inst,
         }
     }
 
-#if WASM_ENABLE_REF_TYPES != 0
-    wasm_runtime_prepare_call_function(exec_env, func);
-#endif
-
     ret = aot_call_function(exec_env, func, argc, argv);
 
-#if WASM_ENABLE_REF_TYPES != 0
-    wasm_runtime_finalize_call_function(exec_env, func, ret, argv);
-#endif
-
     /* don't destroy the exec_env if it's searched from the cluster */
     if (!existing_exec_env)
         wasm_exec_env_destroy(exec_env);