Copy on write using weakrefs (part 2)
Continuation of #11500 

@shoyer @jreback Ballpark timeline on refactor? Like, >1.5 years? 

If so, I think I'll set this aside.Trying to modify the DataFrame constructor just opened a huge can of worms I think it'd take a long time to work through. 

Till the refactor, any interest in making `SettingWithCopyWarning` into an exception?
 I'd like to get it done in under a year; in combing through the codebase I've been making lists of things to do. It's a large effort but luckily we have a huge test suite to protect us and help keep the train on the rails. Will keep you posted
 @wesm Great. Then will def set aside. :)
 @nickeubank thanks for all the effort / commentary tests. I think these will be useful in the refactor.
 @jreback thanks for the guidance! If nothing else, I now know a lot more about how references work in Python! :)
 @nickeubank If you want to apply that knowledge for some internet karma, [this](http://stackoverflow.com/questions/34884536/what-is-the-point-of-views-in-pandas-if-it-is-undefined-whether-an-indexing-oper) came up yesterday ;)
 @TomAugspurger on it, thanks Tom. 
 Is there an open issue for this? I found a bunch of issues related to this, but didn't see any open ones.
 there are refeneced issues
 See https://github.com/pydata/pandas-design/issues/10
