[swift-evolution] Mutability for Foundation types in Swift

[Ben Rimmington]

<https://github.com/apple/swift-evolution/blob/master/proposals/
0069-swift-mutability-for-foundation.md>

The proposal looks great.

## Introduction

Broken link:

-- <https://developer.apple.com/videos/play/wwdc2015-414/>
++ <https://developer.apple.com/videos/play/wwdc2015/414/>

### New Value Types

CharacterSet:

* Rename to UnicodeScalarSet?
* Update APIs to follow SE-0059 (SetAlgebra) proposal?
* Add `enumerateRanges` method, similar to NSIndexSet?

-- Ben