We should not have GeneticCode <: AbstractDict{UInt64, AminoAcid}

The issue is that it exposes the UInt encoding of codons, which is an implementation detail. Also, GeneticCode is still private. That whole API should probably be re-thought; we may keep the translate functions and change how users interact with GeneticCode instances. Also, should users be able to make their own GeneticCode?

Is this related to the interfaces question at all? #328