Concatify: less garbage, less copying, better complexity

gistfile1.lua

    if data then
        local where = {}
        local k, v = next(data, nil)
        if k then
            insert(where, concat{k, '=', sql_tostring(v)})

            for k, v in next, data, k do
                insert(where, concat{' AND ', k, '=', sql_tostring(v)})
            end
        end

        query = concat{query, ' WHERE ', concat(where)}
    end