Added _demo/go/unique with code demonstrating unique.Make usage.
This demo showcases the fix for the compilation panic issue.
Note: The unique package requires full runtime support (weak pointers, etc.)
which is still under development. The test_demo.sh script should be
updated separately to skip this demo on Go 1.23 or earlier.
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Co-authored-by: cpunion <cpunion@users.noreply.github.com>
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