Support a dispatch macro to avoid the need of branching

[Tortar]

E.g. say we have

using MixedStructTypes

abstract type AbstractShape end

@sum_structs Shape <: AbstractShape begin
    @kwdef struct Square
        side::Float64 = rand()
    end   
    @kwdef struct Rectangle
        width::Float64 = rand()
        height::Float64 = rand()
    end
    @kwdef struct Triangle
        base::Float64 = rand()
        height::Float64 = rand()
    end
    @kwdef struct Circle
        radius::Float64 = rand()
    end
end

currently we can define an area function like so:

function area(sh)
    if kindof(sh) === :Square
        area_square(sh)
    elseif kindof(sh) === :Rectangle
        area_rectangle(sh)
    elseif kindof(sh) === :Triangle
        area_triangle(sh)
    elseif kindof(sh) === :Circle
        area_circle(sh)
    else
        area_default(sh)
    end
end
   
area_square(s) = s.side * s.side
area_rectangle(r) = r.width * r.height
area_triangle(t) = 1.0/2.0 * t.base * t.height
area_circle(c) = π * c.radius^2
area_default(::AbstractShape) = 0.0

but actually we can generate all of this. So probably something like this should suffice

@dispatch area(s::Square) = s.side * s.side
@dispatch area(r::Rectangle) = r.width * r.height
@dispatch area(t::Triangle) = 1.0/2.0 * t.base * t.height
@dispatch area(c::Circle) = π * c.radius^2
@dispatch area(::AbstractShape) = 0.0