Prep 0.13.0 release: separate Entry traits from core ones

[jsdw]

• Separate the iterating over entries from the core frame-decode *Info traits; One only needs to implement *Info traits to work with frame-decode; the other traits are convenience traits.
• Make Entry type generic so that it can potentially be used in more places, expose it, and expose concrete versions from each module.
• Expose Kusama RC and AH types (though keep these hidden until the types are more complete).