TST: corner_case for read_csv with usecols GH5766
BUG: regression in read_csv parser handling of usecols GH5766

GH4406 fixed one bug and introduced (exposed?) this corner case
where the loop over columns continues even after all usecols
have been fulfilled, causing an out-of-bound lookup in names.
Merge pull request #5770 from y-p/GH5766

BUG: regression in read_csv parser handling of usecols GH5766
Start of RangeIndex implementation
More work on RangeIndex (with cleaned up tests)
add note on slice locs for non-monotonically increasing Index objects
Flesh out RangeIndex test cases (including translating many of the Int64Index tests to be RangeIndex tests instead)
Add specialized arrmap and groupby for RangeIndex to algos
More work on the RangeIndex implementation
tweaks to test code to get everything working right + block some of the ndarray interface
More fixes to backfill (you know, using something that can actually be filled...shocker)