Higher-level API for unsafe_copy3d!

It would be nice to have higher level API to perform 3-D memcopy (relying on `Mem.unsafe_copy3d!` from `CUDAdrv`). Here is a collection of the discussions on done in the PR [Wrap 3d memcpy JuliaGPU/CUDAnative.jl#238 in CUDAdrv](https://github.com/JuliaGPU/CUDAdrv.jl/pull/238):

> Thoughts on a more high-level API are welcome too.

What do you think about a CUDAnative function with a signature approximately like the following?
```
copy3d!(
src::AbstractArray{},
srcranges::Tuple{Union{UnitRange{},Integer},Union{UnitRange{},Integer},Union{UnitRange{},Integer}},
dst::AbstractArray{},
dstranges::Tuple{Union{UnitRange{},Integer},Union{UnitRange{},Integer},Union{UnitRange{},Integer}};
async::Bool=false,
stream::Union{Nothing,CuStream}=nothing
)
```
Moreover, CuArrays could then maybe invoke this function automatically for 3-D copies in vectorized notation (e.g. A[:, :] = B[:, 2, :]) that match the requirements.

>> What do you think about a CUDAnative function with a signature approximately like the following?

> The problem is that we can't know from an array if it's host|device, or unified memory.

I see. Maybe something along the following lines? That is, the idea would be to use multiple dispatch with specialization for CuArray arguments. For CuArrays one could determine if it is a host/device/unifiedmem array; for the AbstractArrays one would assume always that it is a host array.
```
copy3d!(
src::CuArray{},
srcranges::Tuple{Union{UnitRange{},Integer},Union{UnitRange{},Integer},Union{UnitRange{},Integer}},
dst::CuArray{},
dstranges::Tuple{Union{UnitRange{},Integer},Union{UnitRange{},Integer},Union{UnitRange{},Integer}};
async::Bool=false,
stream::Union{Nothing,CuStream}=nothing
)

copy3d!(
src::CuArray{},
srcranges::Tuple{Union{UnitRange{},Integer},Union{UnitRange{},Integer},Union{UnitRange{},Integer}},
dst::AbstractArray{},
dstranges::Tuple{Union{UnitRange{},Integer},Union{UnitRange{},Integer},Union{UnitRange{},Integer}};
async::Bool=false,
stream::Union{Nothing,CuStream}=nothing
)

copy3d!(
src::AbstractArray{},
srcranges::Tuple{Union{UnitRange{},Integer},Union{UnitRange{},Integer},Union{UnitRange{},Integer}},
dst::CuArray{},
dstranges::Tuple{Union{UnitRange{},Integer},Union{UnitRange{},Integer},Union{UnitRange{},Integer}};
async::Bool=false,
stream::Union{Nothing,CuStream}=nothing
)

copy3d!(
src::AbstractArray{},
srcranges::Tuple{Union{UnitRange{},Integer},Union{UnitRange{},Integer},Union{UnitRange{},Integer}},
dst::AbstractArray{},
dstranges::Tuple{Union{UnitRange{},Integer},Union{UnitRange{},Integer},Union{UnitRange{},Integer}};
async::Bool=false,
stream::Union{Nothing,CuStream}=nothing
)
```

>> For CuArrays one could determine if it is a host/device/unifiedmem array; for the AbstractArrays one would assume always that it is a host array

> No, that's not true, because of unsafe_wrap!.