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.c

@@ -265,6 +265,7 @@ aot_create_import_funcs(const WASMModule *module)
     import_funcs[i].func_name = import_func->field_name;
     import_funcs[i].func_ptr_linked = import_func->func_ptr_linked;
     import_funcs[i].func_type = import_func->func_type;
+    import_funcs[i].signature = import_func->signature;
     /* Resolve function type index */
     for (j = 0; j < module->type_count; j++)
       if (import_func->func_type == module->types[j]) {