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ddnet: Introduce ddnet `DemoWriter` #268

Job	Run time
Check that generated files match	3s
Check that everything is rustfmtted	4s
Build and test (ubuntu-latest, stable)	3m 11s
Build and test (macos-latest, stable)	3m 46s
Build and test (windows-latest, stable)	6m 8s
Build and test (ubuntu-latest, 1.63.0)	4m 38s
Build and test (ubuntu-latest, nightly)	2m 32s
	20m 22s

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