Fix sub_materialize for GPU arrays

[mtfishman]

Currently, sub_materialize (through sub_materialize_axes) falls back to materializing on CPU. This PR generalizes that logic by determining the output destination with similar, which helps to support non-Array types like GPU arrays. As a stand-in for other GPU arrays, I test this using JLArrays.JLArray, which is a reference implementation for the GPUArrays.jl interface that runs on CPU.

An alternative design would be to define memory layouts for GPU arrays (i.e. #9), which would allow more customizability for GPU array backends, however I think it is helpful to have fallbacks that "just work" if reasonable parts of the Base AbstractArray interface are implemented.

I hit this issue because I was testing out BlockArrays.BlockedArray wrapping a GPU array and noticed that calling A[Block(1, 1)] to access a block instantiated the block on CPU, this PR fixes that issue.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.52%. Comparing base (b9b424e) to head (fa93860).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #261   +/-   ##
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  Coverage   89.52%   89.52%           
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  Files          11       11           
  Lines        1938     1938           
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  Hits         1735     1735           
  Misses        203      203           

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