Jonas Jenwald 2fe9f3ff8f Add caching to reduce the number of Ref objects
This is similar to the existing caching used to reduced the number of `Cmd` and `Name` objects.
With the `tracemonkey.pdf` file, this patch changes the number of `Ref` objects as follows (in the default viewer):

|          | Loading the first page | Loading *all* the pages |
|----------|------------------------|-------------------------|
| `master` | 332                    | 3265                    |
| `patch`  | 163                    | 996                     |
2019-05-26 12:23:37 +02:00
..
2019-04-07 13:14:03 +02:00
2018-08-05 21:21:46 +02:00
2017-02-27 08:32:39 -06:00