Reduce DataFrame by unique value

General Usage
2 
julia> using DataFrames

julia> df = DataFrame(id  = [2, 4, 5], value = [3, 3, 1])
3×2 DataFrame
 Row │ id     value
     │ Int64  Int64
─────┼──────────────
   1 │     2      3
   2 │     4      3
   3 │     5      1

julia> combine(groupby(df, :value), :id => Ref)
2×2 DataFrame
 Row │ value  id_Ref
     │ Int64  SubArray…
─────┼──────────────────
   1 │     1  [5]
   2 │     3  [2, 4]
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