[swift-evolution] Take 2: Stdlib closure argument labels and parameter names

[Dave Abrahams]

I've updated my pull request with a much more conservative set of
changes that preserves/restores label-free-ness for all “term of art”
functional methods such as filter and reduce.

https://github.com/apple/swift/pull/2981

My current thoughts are that many of the `by:` labels are awkward and
not adding much.  Perhaps they all ought to be omitted.

-- 
Dave