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()

samples/simple/src/ext_lib_export.c

@@ -1,12 +0,0 @@
-#include "lib_export.h"
-#include "sensor_native_api.h"
-#include "timer_native_api.h"
-#include "req_resp_native_api.h"
-#include "connection_native_api.h"
-
-static NativeSymbol extended_native_symbol_defs[] = {
-#include "runtime_sensor.inl"
-#include "connection.inl"
-};
-
-#include "ext_lib_export.h"