[SR-10288] String should have an initializer that provides mutable access to its contents

[Catfish-Man]

Previous ID	SR-10288
Radar	rdar://problem/49562522
Original Reporter	@Catfish-Man
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Improvement
Assignee	@Catfish-Man
Priority	Medium

md5: dffd3dce1ab3f06ce32234927af3985e

blocks:

• SR-10556 _foreignGrow should use the uninitialized-buffer String initializer once it's in

Issue Description:

For NSString->String bridging we'd like to "eagerly" bridge some NSStrings, and the most promising way of accomplishing this is copying their contents directly into a Swift String. With existing API this requires copying into a temporary buffer, then initializing with the buffer. We can do better 🙂