Merge pull request #3589 from apple/closure-naming-and-labeling
See https://github.com/apple/swift-evolution/blob/master/proposals/0118-closure-parameter-names-and-labels.md
* <label> body => _ body
* _ isSeparator: => whereSeparator isSeparator:
* isOrderedBefore: => by areInIncreasingOrder:
* _ includeElement/whereElementsSatisfy predicate => _ isIncluded
* initialValue: => makingValueWith factory: (ManagedBuffer)
* first/contains(predicate) => first/contains(where: predicate)
* isEquivalent: => by areEquivalent:
* reduce(_ initial:combine:) => reduce(_ initialResult:_ nextPartialResult)
* encode(_:output:) => encode(_:into processCodeUnit:)
