[ base ] Data.SortedSet.leftMost and .rightMost

[MithicSpirit]

Description

Implement leftMost and rightMost for SortedSet in terms of the
functions with the same name in Data.SortedMap.

Should this change go in the CHANGELOG?

• If this is a fix, user-facing change, a compiler change, or a new paper
  implementation, I have updated CHANGELOG_NEXT.md (and potentially also
  CONTRIBUTORS.md).

[MithicSpirit]

Now that I think about this again, it might be better to call these minimum
and maximum since they correspond to the actual value that is returned. This
makes it more in-line with Data.List (which is missing maximum; working on
that now) rather than Data.SortedMap, which I think is preferable since the
use case of sets is closer to that of lists than that of maps. Please let me
know if you would like this change.