`keys(vnt)` should not recurse into PartialArrays if...

[penelopeysm]

... all(pa.mask) is true AND it doesn't contain ALBs/nested PAs/nested VNTs) AND it isn't a GrowableArray

In other words:

julia> using DynamicPPL

julia> vnt = @vnt begin
           @template x = zeros(2)
           x[1] := 1.0
           x[2] := 2.0
       end
VarNamedTuple
└─ x => PartialArray size=(2,) data::Vector{Float64}
        ├─ (1,) => 1.0
        └─ (2,) => 2.0

julia> keys(vnt)
2-element Vector{VarName}:
 x[1]
 x[2]

The final call should return x rather than x[1] and x[2].

[penelopeysm]

Having thought about this quite a lot, I don't think we should change the behaviour of keys, since that entails changes to behaviour of values, map_pairs!!, map_values!!, mapreduce, etc etc.

Furthermore, it is inherently type unstable to check all(pa.mask) and convert the result's type based on that.

I think it's better to introduce a separate function, densify(::VarNamedTuple), that collapses all complete PAs into their normal arrays (subject to the criteria listed above again, i.e., no nested PAs/VNTs/ALBs). Then inside the ParamsWithStats constructor we can use that.

Note that densify needs to go from the bottom up, because we want to catch the case where we have a full PA which itself contains full PAs.