Set NSLayoutConstraint.identifier

[robb]

To make debugging simpler, we could set the NSLayoutConstraint.identifier, maybe we can even include a reference to its layout block (filename and line).

[aranasaurus]

My initial thought was that it would be really nice if there were an operator like the ~ used for priority that would allow us to specify the identifier per statement.

Doing automatically with filename and line is also an interesting concept.