Fix error with ByRow on import

[pdeffebach]

fixes #365

[pdeffebach]

Requesting a quick review. I went and added $ in front of all ByRow and AsTable calls. I also special-cased AsTable so that AsTable on the right-hand-side, such as a row-sum, works even if DataFramesMeta is only imported.

@bkamins

julia> import DataFramesMeta

julia> import DataFrames

julia> df = DataFrames.DataFrame(x1 = 1, x2 = 2, x3 = 3);

julia> DataFramesMeta.@rtransform df :z = sum(AsTable([:x1, :x2, :x3]))
1×4 DataFrame
 Row │ x1     x2     x3     z     
     │ Int64  Int64  Int64  Int64 
─────┼────────────────────────────
   1 │     1      2      3      6

I figures this is okay since we "own" the expression we are operating on. But long term, it might be nice to make this something DataFramesMeta.jl owns more concretely.

[bkamins]

looks OK

[pdeffebach]

Failures on nightly are unrelated. We should merge.