Number of steps may exceed range of Natural

[Jellix]

Describe the bug
See #29 (comment)

Current suspicion is that the reported number of steps exceeds the 2**31 mark (i.e. > Natural'Last). Steps should be recorded in a 64 bit integer.

Expected Behavior
Not a Constraint_Error.

[Jellix]

Apparently, we can have negative step values.

That's a bit of a bummer, as using that as a raw value would throw off the sorting.