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

[Anton Zhilin]

I think current -ed/-ing convention is very ugly. It breaks syntactic correctness, experience from other languages, mathematical notation and functional idioms.

`InPlace` suffix was so far more correct in these terms. We can make anything a convention, but should we?
I liked the proposal about new naming conventions, but overlooked this change.

I think, many people will agree with me. This still can be reviewed until Swift 3.
If so, I will create a proposal to correct "the big three" in this.