Extend map macro to constrained maps

[Christian Schilling]

@map can only read unconstrained maps so far. But we can read constraints in @system. It should be rather easy to share that code (I hope).

[Marcelo Forets]

This is also a source of bugs, e.g. in the following code, the struct is built, but the guard is ignored (!):

julia> guard_1 = Universe(3);

julia> t1 = @map(h -> h, dim: 3, h ∈ guard_1)
IdentityMap(3)