type trait alias templates are needlessly forcing the instantiation of class templates

[ericniebler]

The cuda::std::decay_t type alias is currently defined as:

template <class _Tp>
using decay_t = typename decay<_Tp>::type;

This forces the instantiation of the cuda::std::decay class template, which is not necessary when the __decay compiler intrinsic is available.

#if defined(_CCCL_BUILTIN_DECAY)
template <class _Tp>
using decay_t = _CCCL_BUILTIN_DECAY(_Tp);
#else
template <class _Tp>
using decay_t = typename decay<_Tp>::type;

There are similar efficiency wins to be had for many of the other type trait alias templates.