feat: finish cdylib ffi and improve ffi C header

wfassoc-cdylib/src/object_pool.rs

@@ -17,6 +17,14 @@ pub enum Error {
 /// The token for fetching object in [ObjectPool].
 pub type Token = u64;
 
+/// Get the invalid token.
+/// 
+/// Invalid token is always invalid for fetching object in pool,
+/// And can be useful in FFI scenario.
+pub fn invalid_token() -> Token {
+    DefaultKey::null().data().as_ffi()
+}
+
 /// A pool for managing objects with unique tokens.
 /// 
 /// It is highly suggested to use this pool with [std::sync::RwLock] guard.