SE-0263: Add a String Initializer with Access to Uninitialized Storage

[jawbroken]

@lukasa

On the naming front, I think it's probably a good idea to include the word "unsafe" somewhere in this method name or in the labels.

The similar function for Array uses “unsafeUninitializedCapacity” as the argument label, so this proposal should probably match that or explain why the label should be different.