Replace use of thrust::identity by cuda::std::identity

[bernhardmgruber]

thrust::identity<T> should eventually be replaced by cuda::std::identity. However, contrary to most other function objects, which have the same behavior in Thrust and libcu++, this is not straight forward, since thrust::identity<T> casts whatever argument it gets to const T&. Thus, at every use of thrust::identity<T> it has to be considered whether a cast is intended/required here.

[bernhardmgruber]

We can probably not replace it directly, since thrust::identity<T> performs a cast to T, whereas cuda::std::identity just forwards. But because this behavior is unexpected, we may want to add a function object that casts (with a different name) and deprecate (with eventual removal) thrust::identity.

[bernhardmgruber]

We can probably not replace it directly, since thrust::identity<T> performs a cast to T, whereas cuda::std::identity just forwards. But because this behavior is unexpected, we may want to add a function object that casts (with a different name) and deprecate (with eventual removal) thrust::identity.

Or we just drop it regardless, and users who relied on the casting behavior should supply their own function object which performs a cast.