Add activate/deactivate extension for Constraint.

[kitwtnb]

Before

let constraint: Constraint? = view.leadingToSuperview()

...

if let c = constraint {
    Constraint.deactivate([c])
}

After

let constraint: Constraint? = view.leadingToSuperview()

...

constraint?.deactivate()

[paulocoutinhox]

merge merge merge +1