Fix RangeIndex slicing

[benbovy]

• Closes slicing DataArray with RangeIndex coordinate can put coordinate in inconsistent state #10441
• Tests added

[dcherian]

One trick I have learned is to use range objects to outsource this logic to CPython (e.g. #7586)

So you could do:

newrange = range(self.size)[sl]
start = newrange.start
step = newrange.step or 1
size = len(newrange)

[benbovy]

Nice trick, this also allows supporting reverse indexing without additional effort.

[dcherian]

Actually, I remember the real API is slice.indices(size) so an alternative is :

        start, stop, step = slicer.indices(self.size)
        size = len(range(start, stop, step))

I think you still need to construct the range object to get the right size so not much difference