Add open interval iterators for numbers about motoko-base HOT 7 OPEN

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.

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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

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

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And where should these live? In each integral type?

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

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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.

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enumUntil and enumUpTo?

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

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