Sort `RangeCumsum` for `AbstractUnitRange`

[jishnub]

This makes sort work for infinite RangeCumsums:

julia> r = RangeCumsum(-2:∞)
ℵ₀-element RangeCumsum{Int64, InfiniteArrays.InfUnitRange{Int64}} with indices OneToInf():
 -2
 -3
 -3
 -2
  0
  3
  7
 12
 18
  ⋮

julia> sort(r)
vcat(5-element Vector{Int64}, ℵ₀-element RangeCumsum{Int64, InfiniteArrays.InfUnitRange{Int64}} with indices OneToInf()) with indices OneToInf():
 -3
 -3
 -2
 -2
  0
  3
  7
 12
 18
  ⋮

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.97%. Comparing base (4aa8264) to head (2b673e2). Report is 8 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #224 +/- ## ========================================== + Coverage 89.58% 89.97% +0.39% ========================================== Files 11 11 Lines 1844 1896 +52 ========================================== + Hits 1652 1706 +54 + Misses 192 190 -2 ```

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