Introduce custom ASCII string type for all source manipulation

[ktraunmueller]

With the recent improvements to Swift's String type (UTF8 storage, isAscii performance flag, etc), it seems about time to replace all uses of NSString and NSMutableString.

This could potentially become a huge refactoring, as NSString and NSRange are everywhere. Maybe we should introduce a custom type ASCIIString (backed by a String or gasp! maybe even a ContiguousArray<CChar>?) that provides Int- and NSRange-based indexing and slicing similar to NSString (but assuming ASCII-only strings). This way, most of the existing code wouldn't need to change.

Note: for any code dealing with Unicode text (e.g., input transformation code), expect subtle issues because of differences how NSString and String report the length of a string: NSString.length is the number of UTF-16 code units, while String.length returns the number of perceived characters.

Example (insertedText is an NSMutableString):

(lldb) po insertedText
À
(lldb) po insertedText.length
2

This shouldn't be a problem in most of the code, though, because it's ASCII except for the input stage.

Performance-related: be careful / verify if SourceModel's texSource is referenced only by SourceModel, to prevent triggering creating copies (CoW) when modifying the string.

Prerequisites: #468

[ktraunmueller]

Done in 1.11