Multi-column in anonymous functions

[pdeffebach]

This has numerous threads but not one central one.

We want to be able to have

function f(t::NamedTupleofVectors)
    reduce(+, t)
end

@transform(df, y = f(AsTable([:A, :B]))

this then lowers to

transform(df, AsTable([:A, :B]) => f => :y)

We could ideally do the same with AsVector, where f acts on a Vector of Vectors, or even AsDataFrame.

Not sure how to implement this, though.

[pdeffebach]

Fixed in #307