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

[Brent Royal-Gordon]

> Totally agree with Brent, too. And I wouldn't rename flatten either.

A brief postscript on terms of art:

I wrote my first Python this week. I was very glad that `map` was named `map`. I just had to glance at the docs to see whether it was a function or a method and which argument came first. It took less time and involved less doubt than learning their closure syntax did.

-- 
Brent Royal-Gordon
Architechies