wrap warp intrinsics to fix depricated warnings (#456)

CUDA 9 introduced new, fine-grained, thread synchronization primitives.
In doing so, it introduced new forms of the warp intrinsics like __shfl_up, depricating the old symbols in the process.

It will be a while before we can use 9 as the default minimum, so we have to support compilers that expect the new and old behavior.

There are two options: wrap the intrinsics in question, or pass nvcc a flag to not issue warnings about depricated symbols. I go for the approach of wrapping, because I would rather keep the compiler warning turned on.

Fixes #379.