Extend and generalize allowscalar to memory transfers

Idea from Elliot: a mode for asserting or warning about memory transfers, which are often unwanted. E.g., `CuArrays.allowtransfer(:Bool)`, `CuArrays.@allowtransfer`, `CuArrays.@asserttransfer`.

We should also think about which packages export these functions. They can live in GPUArrays, like `allowscalar` does right now, but I feel like users shouldn't have to import any other package except CuArrays (cf. not having to import CUDAdrv in https://github.com/JuliaGPU/CuArrays.jl/issues/179).