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]
b9ff86e27f