KeyedEncoding: Add new string formatters, capitalized, lowercased, up…

…percased

Some helpers to deal with converting auto-generated codable String values for instance variables to match some common XML key coding standards to the commonly used Swift camel casing

- capitalzied: convert first letter to uppercase
- uppercased: All letters uppercased
- lowercased: All letters lowercased

Support all types that conform to StringProtocol rather than just String

Sources/XMLCoder/Auxiliaries/String+Extensions.swift

@@ -7,7 +7,7 @@
 
 import Foundation
 
-extension StringProtocol where Self.Index == String.Index {
+extension StringProtocol where Self.Index == String.Index  {
     func escape(_ characterSet: [(character: String, escapedCharacter: String)]) -> String {
         var string = String(self)