`hash` definition is not correct

x-ref: https://github.com/JuliaMath/IntervalSets.jl/pull/28#issuecomment-1513607361

```julia
julia> using IntervalSets

julia> hash(1..2)
0x3932a21170706f90

julia> hash(OpenInterval(1,2)) # should be different
0x3932a21170706f90

julia> isequal(2..1, 3..1)
true

julia> unique([2..1, 3..1]) # should be 1-element
2-element Vector{ClosedInterval{Int64}}:
2 .. 1
3 .. 1
```