Add Util.Extract_Unsigned_16 function (proved)
Use Unsigned_8 instead of own defined 0..255 for Octet type
Minor layout fixes on requirements. Add REQ-5 to REQ-6.6
Add Parse.Parse_Header procedure, implementing REQ-6 to REQ-6.6

* Procedure is proved, except bit shifting operations not supported by
  GNATprove.

* Not tested.