Remove generic from RawLua::push_userdata_metatable.

This should help to reduce amount of generated code.

src/userdata/util.rs

@@ -1,5 +1,9 @@
 use std::cell::Cell;
 use std::marker::PhantomData;
+use std::os::raw::c_int;
+
+use super::UserDataStorage;
+use crate::util::{get_userdata, take_userdata};
 
 // This is a trick to check if a type is `Sync` or not.
 // It uses leaked specialization feature from stdlib.
@@ -29,3 +33,14 @@ pub(crate) fn is_sync<T>() -> bool {
     .clone();
     is_sync.get()
 }
+
+pub(super) unsafe extern "C-unwind" fn userdata_destructor<T>(state: *mut ffi::lua_State) -> c_int {
+    let ud = get_userdata::<UserDataStorage<T>>(state, -1);
+    if !(*ud).is_borrowed() {
+        take_userdata::<UserDataStorage<T>>(state);
+        ffi::lua_pushboolean(state, 1);
+    } else {
+        ffi::lua_pushboolean(state, 0);
+    }
+    1
+}