'substring(to:)' is deprecated: Please use String slicing subscript with a 'partial range upto'

swift3 消除警告'substring(to:)' is deprecated: Please use String slicing subscript with a 'partial range upto' 

Swift3废除了subString(from\to\with:)字符串截取方法。

而截取方法改成了：

let newStr = String(str[..<index]) // = str.substring(to: index) In Swift 3

let newStr = String(str[index...]) // = str.substring(from: index) In Swif 3

let newStr = String(str[range]) // = str.substring(with: range) In Swift 3

可以给string写个扩展延续之前的方法