Cleanup & remove some deps

[pxl-th]

• Get rid of Requires.
• Remove GPUArrays dep as it is not used anywhere.
• Move Atom to extension.
• Minor cleanups after dropping 1.6 support.
• Remove Zygote.refresh() as it seems it is no longer required (closes Seems to be #265ing #22):

julia> using Zygote

julia> g(x) = 2x;

julia> @show g'(10);
(g')(10) = 2.0

julia> g(x) = 3x;

julia> @show g'(10);
(g')(10) = 3.0

[ToucheSir]

#1272 is why the GPUArrays compat bound was put in place. As long is the downgrade issue there is no longer a problem, I think we can remove it. Otherwise this LGTM.

[pxl-th]

#1272 is why the GPUArrays compat bound was put in place. As long is the downgrade issue there is no longer a problem, I think we can remove it. Otherwise this LGTM.

Shouldn't be a problem. All GPU backends now use [email protected] and that itself will require newer GPUArrays package.