Refactor factory functions

Remove $littleEndian parameter
Add fromGuidString/Bytes factory methods for GUIDs
No longer possible to output GUIDs as UUIDs and vice-versa
Pass tests

src/UuidInterface.php

@@ -9,6 +9,8 @@ interface UuidInterface
 
     public function equals($other);
 
+    public function getHex();
+
     public function getFieldsHex();
 
     public function getClockSeqHiAndReservedHex();