Rust

Introduce the `PREV_SNAPSHOT` flag #371

Job	Run time
Check all the features of the heed3 project (ubuntu-latest)	14s
Check the heed3 project (ubuntu-latest)	16s
Run the heed examples (ubuntu-latest)	24s
Check the heed3 project (windows-latest)	1m 34s
Run the heed examples (macos-latest)	44s
Check the heed3 project (macos-latest)	29s
Check all the features of the heed project (ubuntu-latest)	15s
Check all the features of the heed project (macos-latest)	32s
Check all the features of the heed3 project (macos-latest)	35s
Ensure the heed project is formatted	14s
Run the heed3 examples (ubuntu-latest)	21s
Run the heed3 examples (macos-latest)	38s
Ensure heed3 is not erasing heed	6s
Test the heed project (ubuntu-latest)	36s
Test the heed project (windows-latest)	1m 47s
Test the heed project (macos-latest)	44s
	9m 29s

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