stdlib: Add __repr__ to pystats
For Statistics the value is returned. E.g.:
```
print(simstats.board.core.some_integer)
> 5
```
For Groups the names of the stats in that group are listed.
E.g.:
```
print(stats.board.core)
> [Group: [some_integer, another_stat, another_group]]
```
Change-Id: I94cea907608fba622f4fc141d5b22ac95d8cde40
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/63271
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
2 files changed
