Shorthand for Offsetting startIndex and endIndex

[lorentey (Karoy Lorentey)]

@DeFrenZ

The argument against the subscript is exactly this: I believe that in general, at least in the standard library, properties and subscripts are always O(1).

String is an important exception, because Unicode makes things complicated.

• String.subscript(i: Index) is not guaranteed to be a constant-time operation, because it sometimes needs to find the end of a grapheme cluster.
• Similarly, String.first/String.last are examples of O(n) properties.