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. Generated with [codeagent](https://github.com/qbox/codeagent) Co-authored-by: cpunion <cpunion@users.noreply.github.com>