Always disallow scalar indexing.

[maleadt]

Fixes JuliaGPU/CUDA.jl#178.

[maleadt]

Update:

julia> using CUDA
julia> cu([1])[1]
┌ Warning: Performing scalar indexing.
│ Invocation of getindex resulted in scalar indexing of a GPU array.
│ This is typically caused by calling an iterating implementation of a method.
│ Such implementations *do not* execute on the GPU, but very slowly on the CPU,
│ and therefore are only permitted from the REPL for prototyping purposes.
│ If you did intend to index this array, annotate the caller with @allowscalar.
└ @ GPUArrays ~/Julia/pkg/GPUArrays/src/host/indexing.jl:57
1
                                                                                                                                                                                                                                  tim@taurus ~/Julia/pkg/GPUArrays$ jl -e 'using CUDA; cu([1])[1]'
ERROR: Scalar indexing is disallowed.
Invocation of getindex resulted in scalar indexing of a GPU array.
This is typically caused by calling an iterating implementation of a method.
Such implementations *do not* execute on the GPU, but very slowly on the CPU,
and therefore are only permitted from the REPL for prototyping purposes.
If you did intend to index this array, annotate the caller with @allowscalar.
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:33
 [2] assertscalar(op::String)
   @ GPUArrays ~/Julia/pkg/GPUArrays/src/host/indexing.jl:54
 [3] getindex(xs::CuArray{Int64, 1}, I::Int64)
   @ GPUArrays ~/Julia/pkg/GPUArrays/src/host/indexing.jl:87
 [4] top-level scope
   @ none:1

@vchuravy