Generated enumerations should be implementing a yang-binding Enumeration interface

Generated enumerations have a getName() method which exposes the YANG-specified assigned name. Unfortunately this method is not captured in an interface contract, which leads to two things:

• we need to generate javadoc over and over again
• codec has to go back to mapping to recover them (or use reflection, ick)

Introduce Enumeration interface in yang-binding and have all enumerations implement it - that way we properly capture the constants  and the codec can reconstruct the mapping easily and just perform sanity checking if needed.