refactor set_ids() to use enumerate and/or list comprehension

[dimitrilw]

When able, change DisjointSet.Set_ids() to use enumerate and/or list comprehension.

enumerate():

...
for i, p in enumerate(self.parents):
     if i == p:
        ...

list comprehension:

return List[Int](i for i, p in enumerate(self.parents) if i == p)

[dimitrilw]

Of note: we can't do this yet. I am going to add a "blocked" tag. This task is blocked, awaiting more features in core Mojo language.