Add UniquePtr::to_shared and SharedPtr::from_unmanaged

[russelltg]

This adds a way to create SharedPtr's from C++ pointers created with new (or via unique_ptrs).

I'm a new contributor here, let me know if I did this in a reasonable way, and I'm happy to put some effort in if this is something that's interesting.

[russelltg]

I fixed that error and added an additional test to cover the situation, but it needs approval to run again. Just pinging just in case you don't get commit notifications.

[russelltg]

Freshly rebased onto master :)

[russelltg]

Hmm I think it would be reasonable also to implement SharedPtr<T> from UniquePtr<T>, happy to add that if you agree.