Added assorted superviewMargins and superviewSafeArea relatable shortcuts

[sgtsquiggs]

Adds relatable methods:

func equalToSuperviewMargins() -> ConstraintMakerEditable
func equalToSuperviewSafeArea() -> ConstraintMakerEditable
func lessThanOrEqualToSuperviewMargins() -> ConstraintMakerEditable
func lessThanOrEqualToSuperviewSafeArea() -> ConstraintMakerEditable
func greaterThanOrEqualToSuperviewMargins() -> ConstraintMakerEditable
func greaterThanOrEqualToSuperviewSafeArea() -> ConstraintMakerEditable

[MauriceArikoglu]

@robertjpayne @sgtsquiggs is it possible to have this PR approved and merged? These convenience methods would really improve the overall readability and workflow of SnapKit dependent projects.

[kmcginnes]

Hi, I just wanted to see if there was a plan to merge these in and get a release out soon-ish? It's been more than a year since this PR was created.

[kmcginnes]

Possibly related to #459