Add drop handler for async cursor

odbc-api/src/cursor.rs

@@ -291,7 +291,7 @@ where
 /// 
 /// Like [`CursorImpl`] this is an ODBC statement handle in cursor state. However unlike its
 /// synchronous sibling this statement handle is in asynchronous polling mode.
-pub struct AsyncCursor<Stmt> {
+pub struct AsyncCursor<Stmt: AsStatementRef> {
     /// A statement handle in cursor state with asynchronous mode enabled.
     statement: Stmt
 }