Make Ref mutable on the GPU.

[maleadt]

As requested by @utkarsh530:

julia> x = Ref(0)
Base.RefValue{Int64}(0)

julia> function kernel(ref)
           ref[] = threadIdx().x
           return
       end
kernel (generic function with 1 method)

julia> @cuda kernel(x)
CUDA.HostKernel for kernel(CUDA.CuRefValue{Int64})

julia> x
Base.RefValue{Int64}(1)

I'm not convinced we want this, because it requires additional API operations when launching a kernel (to pin the memory). Right now, we are essentially passing the Ref as a Tuple, which doesn’t require any API operations, but makes it immutable of course. It's for the same reason that we don't support nested containers.

Fixes #267

[maleadt]

Looking into the failures, the problem is that Ref is also used in the context of broadcast as e.g. RefValue{CuArray}, which has to be materialized as RefValue{CuDeviceArray}. That however breaks the ability to replace the Ref's own memory...

So we need to do something different based on the assumed user's intent. Either we differentiate based on isbits or not, or we only do the Ref content conversion in the context of broadcast...

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (6a8293b) 72.34% compared to head (c062cfd) 60.49%.

❗ Current head c062cfd differs from pull request most recent head f0d00cd. Consider uploading reports for the commit f0d00cd to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2109       +/-   ##
===========================================
- Coverage   72.34%   60.49%   -11.85%     
===========================================
  Files         159      154        -5     
  Lines       14513    14043      -470     
===========================================
- Hits        10500     8496     -2004     
- Misses       4013     5547     +1534     

Files	Coverage Δ
src/compiler/execution.jl	87.07% <86.66%> (-0.34%)	⬇️
lib/cudadrv/memory.jl	77.37% <0.00%> (-1.82%)	⬇️

... and 57 files with indirect coverage changes

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