Add conversion form UniquePtr<T> to SharedPtr<T>

[apbr]

In C++ you can convert a unique_ptr to a shared_ptr using this constructor:

template<class Y, class Deleter>
shared_ptr(std::unique_ptr<Y, Deleter>&& r);

(source)

If we could do such a conversion in Rust this could at least alleviate the issue described in #885 a bit.
Is there any reason not to add it?

[apbr]

Seems like there is already a PR implementing this: #1005