Handlers.Call.Originate: Bug fix and clean-up.

- Fixing bug in parsing of reception contact identifiers.
- Reorganised `Generate_Response`.
- Substituted protocol description with a reference to the protocol specification.
- Terminology update in package `Model`.
- `Image` and `Value` functions for type `Model.Reception_Contact_Identifier`.
- Using `Reception_Contact_Identifier` instead of separate `Contact_Identifier` and `Reception_Identifier`.