Do we need to provide `ordered`?

`Base.minmax` is almost the same, and `IntervalSets.ordered` is not used in IntervalSets.jl itself.

```julia
julia> using IntervalSets

julia> ordered(1,2)
(1, 2)

julia> minmax(1,2)
(1, 2)

julia> ordered(1.0,2)
(1.0, 2.0)

julia> minmax(1.0,2)
(1.0, 2.0)

julia> ordered("bc","ade")
("ade", "bc")

julia> minmax("bc","ade")
("ade", "bc")
```

The only differences are type promotion and if-statement.

```julia
julia> ordered([2.0], [1,3])
([1.0, 3.0], [2.0])

julia> minmax([2.0], [1,3]) # does not promote
([1, 3], [2.0])
```

`IntervalSets.ordered` uses `ifelse`.

https://github.com/JuliaMath/IntervalSets.jl/blob/master/src/IntervalSets.jl#L68

But `Base.minmax` uses ternary operator `_?_:_`

https://github.com/JuliaLang/julia/blob/8922eaa5517536eb51eb542f4543a8c6b82fda34/base/operators.jl#L514

I'm not sure which implementation is ideal, but `Base.minmax` should be fixed if `IntervalSets.ordered` wins in some aspect, and I think we don't have to provide our `ordered` function.