chore: rename kani::visitor to reachability as original code does

Author: zjp-CN

src/functions/kani/mod.rs

@@ -1,4 +1,4 @@
 mod coercion;
-mod visitor;
+mod reachability;
 
-pub use visitor::collect_reachable_items;
+pub use reachability::collect_reachable_items;