Thrust lacks generic malloc/free

[jaredhoberock]

There is no thrust::malloc or thrust::free.

This makes it difficult to allocate (deallocate) tagged memory generically.

WARing this omission forces the user to dip into the private generic namespace and dispatch through ADL.

Consider adding these and exposing them in thrust/memory.h

reported by Bryan Catanzaro

[jaredhoberock]

If these are added, then detail::malloc_allocator should use them instead of the ad hoc dispatching it does.