ENH: Make use of RangeIndex in DatetimeIndex 
`DatetimeIndex` looks like it can benefit from `RangeIndex` #11892 .  
Use case 1: Evenly-spaced time-series analysis
Use case 2: I sometimes (ab)use `date_range` like a PY3 `range` in a for loop, which I think is quite neat.  It would be cooler if it were lazily evaluated.
