Enable bulk selection of columns from multiple tables

[davidanthoff]

After a join one can have multiple tables in the query, and might want to select all columns from all tables. The proposed syntax for this is:

@from i in table1 begin
@join j in table2 on i.a equals j.a
@select {i..., j...}
end

In order to implement this, we need a type stable merge function (JuliaData/NamedTuples.jl#14), which is currently blocked by JuliaLang/julia#16806 on julia 0.5.