Use unsafe_unwrap (or stdlib equivalent) for NonNull

[joshlf]

Now that we're using NonNull instead of *mut, we're sometimes using Option<NonNull<T>> where we would otherwise use a *mut T that could be null. Where we previously would have dereferenced if we already knew that the pointer was non-null, we now have to do ptr.unwrap(), which incurs runtime overhead.

Instead, we should use unsafe_unwrap or a stdlib equivalent if one is ever added (see rust-lang/rust#48278).