[swift-evolution] String update

[Michael Ilseman]

Hello, I just sent an email to swift-dev titled "State of String: ABI, Performance, Ergonomics, and You!” at https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20180108/006407.html, whose gist can be found at https://gist.github.com/milseman/bb39ef7f170641ae52c13600a512782f. I posted to swift-dev as much of the content is from an implementation perspective, but it also addresses many areas of potential evolution. Please refer to that email for details; here’s the recap from it:

### Recap: Potential Additions for Swift 5

* Some form of unmanaged or unsafe Strings, and corresponding APIs
* Exposing performance flags, and some way to request a scan to populate them
* API gaps
* Character and UnicodeScalar properties, such as isNewline
* Generalizing, and optimizing, String interpolation
* Regex literals, Regex type, and generalized pattern match destructuring
* Substitution APIs, in conjunction with Regexes.