Set macro

Task: define convenience macros for modeling with sets.

Related libraries:

• https://github.com/JuliaDiffEq/ModelingToolkit.jl
• https://github.com/JuliaHomotopyContinuation/HCModelKit.jl
• JuMP

For instance,

H = HalfSpace([1., 1.], 2.0) # x + y <= 2

@variables x y
H = @set x + y <= 2