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

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

```c++
template
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.

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

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