Return Iterable<MessagePart> from toParts

[stasm]

If possible in the language of the spec, consider returning an Iterable<MessagePart>? Conforming implementations can still return an array of parts, but some use-cases may benefit from a more lazy approach made possible by the use of generators, for example.

Originally posted by @stasm in #22 (comment)

[eemeli]

Collecting some of my comments on this here from the original thread, so that it may be closed:

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Would that make the error handling potentially weird, though? Let's say that a user does this:

const src = '{n is {$n :number minimumFractionDigits=999}}';
const mf = new Intl.MessageFormat(src, 'en');
const parts = mf.formatToParts({ n: 42 }, console.warn);
for (let part of parts) console.log(part);

With array output, it's pretty clear that the console.warn call will happen before the console.log calls. But with an iterable that's formatting parts lazily, that might log:

{ type: 'literal', value: 'n is ' }
RangeError: invalid digits value: 999
{ type: 'fallback', source: '$n' }

whereas a more eager implementation would report the error earlier.

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I think we do want an array. Not because we may want to read its length or access parts by index, but because:

1. It's clearer to the caller that onError will never be called after the function has returned a value.
2. It's much more user-friendly to allow resetting the position within the parts.

@stasm It would be helpful to have an example or two of use cases which "may benefit from a more lazy approach made possible by the use of generators".