new function: vw_rename_datasets()

[ijlyttle]

As of Altair 2.2, when you concatenate, data for each layer or concatenation are combined into a top-level dataset, reducing duplication. This is a great and wonderful thing.

The name of the resulting dataset seems to be based on the hash of the dataset, and likely for good reason. However, this makes it less than intuitive to work with a "fully cooked" spec because the names are not intuitive.

We could imagine a function:

vw_rename_datasets <- function(spec) {}

That takes a spec, then returns a copy of the spec with the datasets renamed.

The names of the datasets are the names of the top-level dataset object.

They are used as the value of a name string of a data object.