StringIndex - Reasonable indexing into Swift Strings

An experimental package to explore what can be done about Swift's dystopian string indexing. At the moment, you have to perform this memorable dance to get the 5th character of a String:

let fifthChar: Character = str[str.index(str.startIndex, offsetBy: 4)]

This package defines addition, and subtraction operators for the String.Index type returning a temporary enum which conveys the offset and index to subscript operators on StringProtocol which iadvances by the offset lazilly (when it knows the String being indexed). The result of this is you can now get the same result by typing:

let fifthChar: Character = str[.start+4]

There are also range operators and subscripts defined so you can use the following to remove the leading and trailing characters of a string for example:

let trimmed: Substring = str[.start+1 ..< .end-1]

Or you can search in a String for a Character:

let firstWord: Substring = str[..<(.first(of:" "))]
let lastWord: Substring = str[(.last(of: " ")+1)...]

All subscript operators have setters defined so you can modify string contents.