Solidify what KeyedDataset interface

[rofinn]

Related to #25 it's unclear what common Julia operations are applicable to a KeyedDataset (e.g., first(ds), values(ds), etc). Maybe it's worth making this type an AbstractDict with some extra functions?

[nickrobinson251]

Or perhaps behave similarly to a Dictionaries.jl Dictionary, which doesn't subtype AbstractDict, largely because they "can be manipulated and transformed using a similar interface to Julia's built-in arrays", which i think makes sense for a KeyedDataset e.g. mapping and broadcasting preserves the indices and transforms the corresponding values.

[rofinn]

Interesting, I don't love that there's a different type call Dictionary with a different show syntax, but I do like that:

1. setindex! and insert! are distinct with different use-cases.
2. Most of the AbstractIndices behaviour matches most of our set related operations and queries.