Add open interval iterators for numbers

[rossberg]

Currently, range has type (Nat, Int) -> Iter. The Int was introduced by @chenyan-dfinity to safely support patterns like range(0, array.size()-1).

Instead, we should provide variants of range and revRange that interpret the bounds as an open interval on the max end. We probably shouldn't change range itself, though, since it is too natural to (mis)interpret range(0, 10) as a closed interval, which could lead to bugs if it wasn't.

Any ideas for a nice suggestive naming scheme for open range functions?

[kritzcreek]

Here are the trait names Rust uses for these (they also have Syntax for ranges, but maybe the names are helpful anyway):

1..2;   // std::ops::Range
3..;    // std::ops::RangeFrom
..4;    // std::ops::RangeTo
..;     // std::ops::RangeFull
5..=6;  // std::ops::RangeInclusive
..=7;   // std::ops::RangeToInclusive

[nomeata]

Haskell says enumFromTo for the inclusive variant; so maybe Nat.iterFromTo and Nat.iterFromUntil? (Not crystal clear)

[crusso]

And where should these live? In each integral type?

[crusso]

Or can we write a generic one that takes a module (perhaps with min an max values) and does the right thing now?

[rossberg]

Per @nomeata's earlier suggestion, I think it's appropriate if they live in each integral type's module.

As for naming, I'd keep range as is and add rangeBelow for the exclusive version perhaps? Is that good English?

Not sure we desperately need all the open variants.

[nomeata]

enumUntil and enumUpTo?

Should it really be range, or do we want a method name that sounds like a verb?

[rossberg]

We don't use verbs for other iterators, like keys, vals, entries -- I suppose nouns are established practice because they read more naturally with for ... in X. Probably for the same reason, range is more commonly used, AFAICT.