Deref and DerefMut for UniquePtr

[dtolnay]

The impls will panic on a null pointer. Panicking in Deref is unconventional but this matches the reality of how unique_ptr is used in practice in C++. In most places that unique_ptr is passed around they can never be null, and in the few cases where null is allowed the programmer is responsible for keeping track and handling those pointers carefully.

Providing these impls is less bad than users writing ptr.as_ref().unwrap() everywhere, because at least this way we can get the panic message to include type info to help debugging.