Addition and substraction of UnitRanges and StepRanges

[bkamins]

Following the discussion on Slack.

Currently this works :

julia> (1:2) - (1.0:2)
0.0:0.0:0.0

But these operations fail:

julia> (1:2) - (1:2)
ERROR: ArgumentError: step cannot be zero

and

julia> (1:1:2) - (1:1:2)
ERROR: ArgumentError: step cannot be zero

It would be more consistent to allow them, but then internal design of UnitRange and StepRange would need to change (and a tricky part is that the length can span from 0 to 2^(sizeof(type)*8) which is just one value too much to use the same storage type for values and for length).

Additionally we have the following problems with ragne:

julia> a = 2^60
1152921504606846976

julia> a:-a
1152921504606846976:1152921504606846975

julia> BigFloat(a):BigFloat(-a)
ERROR: ArgumentError: length cannot be negative, got -2305843009213693951

[mbauman]

Duplicate of #10391