gh-37904: Change `SetSystem` representation
This is an attempt to represent `SetSystem` in a more informative and
user-friendly way.
Previous:
```
sage: M = matroids.CompleteGraphic(7)
sage: M.bases()
Iterator over a system of subsets
```
Current:
```
sage: M = matroids.CompleteGraphic(7)
sage: M.bases()
SetSystem of 16807 sets over 21 elements
```
I think this is an improvement from resorting to calling, e.g., `len`,
in order to to get some info, after you get struck with `Iterator over a
system of subsets`.
Note that a `SetSystem` is not an iterator but simply an iterable (so
`__repr__` is also misleading). The previous developers were planning to
make it into an _actual_ iterator, as can be seen on [L66](https://githu
b.com/sagemath/sage/blob/f1dc325cf01ac542d42257b77aa322cd92e74de8/src/sa
ge/matroids/set_system.pyx#L66) of `set_system.pyx`.
URL: https://github.com/sagemath/sage/pull/37904
Reported by: gmou3
Reviewer(s): Travis Scrimshaw
