change type of index labels from slice to range

[telamonian]

having the type of the index labels be a list of python slice objects on the python side and an array of ISlice objects on the typescript side is, at best, very confusing. I'm essentially using each "slice" here to define a range of index values, so let's just call these things range (python) and IRange (typescript) instead.

To clarify, I'm proposing simply to change the names of these types, not any behavior