Add Weak

[fredpi]

I suggest adding a Weak wrapper alongside the Mutable wrapper requested here.

Suggested Implementation can be found here.

[Jeehut]

Could you please explain, how this would improve Swift with some usage examples and why you think "it should have been part of the Swift Standard Library"? Also, could you check if there were discussions about this (or something solving the same problem) at the new Swift Evolution Forum and link it here, if you can find it? Thank you in advance.

[fredpi]

In fact there is a discussion on this matter.

While some argue Weak is often useful in many projects, general consensus seems to be it doesn't fit into the standard library, as the "weak reference status" of a property should be a property of this property instead of a new type somehow...

This makes sense, and since I'd be against adding it to the standard library with this in mind, it doesn't fit here either. Closing...