Link `packedsets` in the standard library documentation

The `intsets` module is deprecated in favor of the `packedsets` module. However, the [standard library documentation](https://nim-lang.github.io/Nim/lib.html#pure-libraries-collections) still only links the `intsets` module and not the `packedsets` module. I suggest to add a link to the `packedsets` module there and possibly remove the link to the `intsets` module (since it's deprecated).