Implement to_interval for RangeInclusive, RangeFrom and RangeToInclusive

[George-Ogden]

Implement to_interval for RangeInclusive, RangeFrom and RangeToInclusive
The ranges are inclusive to match the interval
This means that you can do the following:

(1..=5).to_interval()
(1..).to_interval()
(..=5).to_interval()

but the following fail:

(1..5).to_interval()
(..5).to_interval()

Alternative to #16

[ptal]

Thanks.

Why not implementing (1..5) and (..5)? We can decrease by one the upper bound.

[George-Ogden]

I think it is confusing:
1..5 = 1,2,3,4 so it's ambiguous whether 1..5 == [1,5] or 1..5 == [1,4]?

[ptal]

Hmm, yeah, more explicit with (1..=5) is better I guess.