fixes #229: flatMap supports returning both iterables and iterators

[michaelficarra]

Fixes #229.

update: Now also fixes #237, aligning flatMap and from.

Iterator.prototype.flatMap and Iterator.from:

• non-Object: throw
• has Symbol.iterator: call it, get sync iterator, iterate
• has callable "next": assume sync iterator, iterate
• otherwise: throw

AsyncIterator.prototype.flatMap and AsyncIterator.from:

• non-Object: throw
• has Symbol.asyncIterator: call it, get async iterator, async iterate
• has Symbol.iterator: call it, get sync iterator, lift to async iterator, async iterate
• has callable "next": assume async iterator, async iterate
• otherwise: throw

[bakkot]

This logic could probably be shared with Iterator.from and AsyncIterator.from, no? It's not exactly the same as written, but I think it probably should be the same.

(Other than the inheritance check and wrapping logic.)

[michaelficarra]

@bakkot This PR now updates the from methods to share an implementation with flatMap.

[bakkot]

It feels very strange for me to have Iterator.from(string) throw, given that for (let c of string) works. I am (very reluctantly) OK with .flatMap in particular guarding against strings, but I am against Iterator.from doing that guard.

[zloirock]

Strings should be considered as iterables since the current option is inconsistent with all the rest cases of the iterators protocol usage.