Support for kwdef constructors

[ameligrana]

e.g. I imagine a syntax like

@sum_type @kwdef AT begin
    A(common_field::Int = 3, a::Bool = true, b::Int)
    B(common_field::Int = 1, a::Int, b::Float64 = 4.0, d::Complex)
end

and so it will work creating new all keywords constructors for each variant just like the kwdef macro works

julia> A(; b = 4)
A(3, true, 4)::AT

I will try to work out the details if this feature is welcomed

[MasonProtter]

yeah that'd be nice!