Based on 01c96396: core/irrt: add Slice and Range and part of
8f9d2d82: core/ndstrides: implement ndarray indexing.
Needed for implementing general ndarray indexing.
Currently IRRT slice and range have nothing to do with NAC3's slice
and range. The IRRT slice and range are currently there to implement
ndarray specific features. However, in the future their definitions may
be used to replace that of NAC3's. (NAC3's range is a [i32 x 3], IRRT's
range is a proper struct. NAC3 does not have a slice struct).
