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/compilation/aot.h

@@ -83,6 +83,8 @@ typedef struct AOTImportFunc {
   uint32 func_type_index;
   /* function pointer after linked */
   void *func_ptr_linked;
+  /* signature from registered native symbols */
+  const char *signature;
 } AOTImportFunc;
 
 /**