Implement native function pointer check, addr conversion and register, update documents (#185)

Modified WASM runtime API:
- wasm_runtime_module_malloc()
- wasm_runtime_lookup_function()
Introduced runtime API
- wasm_runtime_register_natives()

core/iwasm/common/wasm_exec_env.h

@@ -30,6 +30,10 @@ typedef struct WASMExecEnv {
     /* The WASM module instance of current thread */
     struct WASMModuleInstanceCommon *module_inst;
 
+#if WASM_ENABLE_AOT != 0
+    uint32 *argv_buf;
+#endif
+
     /* Current interpreter frame of current thread */
     struct WASMInterpFrame *cur_frame;