swift234

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:)