[swift-evolution] [SR-933] Rename flatten to flattened

[Brent Royal-Gordon]

> The 'flatten()' method didn't get the Swift 3 API renaming treatment it should have, to go along with reversed, sorted, joined, etc.
> As I see Dmitri Gribenko already agree with it but we still have to discuss it here. So what do you think?

I'm in favor.

Though all of these things are terms of art, not all terms of art are created equal. For instance:

* `map` is supported by virtually any language which have any of these higher-order functions, and to my knowledge the name `map` is universally used.
* `reduce` is not quite as universally supported, but it's still very common, and most (but not quite all) languages with higher-order functions support it.
* `filter` is very widely supported, but the *name* `filter` is not quite so consistent. Ruby, for instance, calls it `select`, Perl calls it `grep`, etc.
* `takeWhile` lies on the other end of the spectrum, being very narrowly supported.

In my opinion, it would be a really bad idea to rename `map` or `reduce`; `filter` is probably a bad idea but not terrible; but we should feel relatively free to rename `takeWhile`.

`flatten` is nowhere near as weak a term of art as `takeWhile`, but I think it still falls towards that end of the spectrum. We shouldn't worry too much about changing it. `map`, `reduce`, and `filter` are much stronger terms, and we should be more cautious about changing them.

-- 
Brent Royal-Gordon
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