Annotate execute_with_allocator's function members __host__ __device__
Add is_allocator trait
Eliminate operator()(Allocator, cudaStream_t) member function
Add .on(cudaStream_t) member function

Fixes #497
Guard cudaGetErrorString with __BULK_HAS_CUDART__
Use new stream syntax in unit tests
Merge branch 'master' of github.com:jaredhoberock/thrust into streams
Merge pull request #530 from jaredhoberock/streams

Change the syntax for requiring CUDA kernel launches on a particular stream