ENH: allow start=range_object in RangeIndex constructor
- [x] closes #12067
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

simplifies `Index.__new__` a bit
hah I originally implemented this but then we settled on `.from_range`. I would be +1 in using this. cc @jorisvandenbossche  I'm +1 on this change. If we call something RangeIndex, it's reasonable that it takes a range in its constructor IMO. (no strong opinion here) lgtm. I think ok to also deprecated `.from_range` In the RangeIndex class doc stringm in the `Parameters` section, I think the line `start : int (default: 0), or other RangeIndex instance` should be changed to `start : int (default: 0), range or other RangeIndex instance` > In the RangeIndex class doc stringm in the Parameters section, I think the line start : int (default: 0), or other RangeIndex instance should be changed to start : int (default: 0), range or other RangeIndex instance

good idea.  will add this to my upcoming core.indexes TYP branch if thats OK thanks @jbrockmendel 

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