swift3.0中String.Index截取字符串方法，很方便哦

Swift3.0 中不能直接使用下标数字进行字符串截取，只能使用String.Index来做位置索引，要想实现截取功能首先得获取到String.Index;

let str = "abcdefs sdfsd"

//计算个数

str.characters.count

//to

let a = str.index(str.startIndex, offsetBy: 3)

str.substring(to:a)  //"abc"

//from

let b = str.index(str.endIndex, offsetBy: -3)

str.substring(from:b)  //"fsd"

//range

let r = Range(uncheckedBounds: (lower:a, upper: b))

str.substring(with:r) //"defs sd"