ENH: TimeDeltaIndex and corresponding scalar
- [x] disallow operations in series that are invalid that now create timedelta64
  (e.g. dt + dt, only dt - dt?) #4534
- [x] create the TimeDeltaIndex object, inherit from Int64, #8184
- [x] create associated scalar (np.timedelta64 wrapper), like
  Timestamp (Timedelta?), direct wrapper might work, #8184
  that forwards methods, internally repr as timedelta64 (or view?) as array
- [x] move pretty printing to the new class (from function in lib), done???
  see #3002 and #2990, #4378
- [x] need a Timedelta64 Block, with to_native_types (for printing), PR #4684
- [x] ~~read_csv to have a `parse_timedelta` to automatically apply the dtype, see #4650, #4684, moved to #8185~~
