auto merge of #13835 : alexcrichton/rust/localdata, r=brson

This commit brings the local_data api up to modern rust standards with a few key
improvements:

* All functionality is now exposed as a method on the keys themselves. Instead
  of importing std::local_data, you now use "key.set()" and "key.get()".

* All closures have been removed in favor of RAII functionality. This means that
  get() and get_mut() no long require closures, but rather return
  Option<SmartPointer> where the smart pointer takes care of relinquishing the
  borrow and also implements the necessary Deref traits

* The modify() function was removed to cut the local_data interface down to its
  bare essentials (similarly to how RefCell removed set/get).

[breaking-change]

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