Do not panic by default

Replace `Row::get` by `Row::get_checked`,
And rename original `Row::get` to `Row::get_unwrap`.
`Stmt::query_map`, `Stmt::query_map_named`, `Stmt::query_row`,
`Conn::query_row` and `Conn::query_row_named` callback parameter must return a `Result`.

src/vtab/csvtab.rs

@@ -389,7 +389,7 @@ mod test {
 
             let mut rows = s.query(NO_PARAMS).unwrap();
             let row = rows.next().unwrap().unwrap();
-            assert_eq!(row.get::<_, i32>(0), 2);
+            assert_eq!(row.get_unwrap::<_, i32>(0), 2);
         }
         db.execute_batch("DROP TABLE vtab").unwrap();
     }