Feature Request: Continuously save progress while shrinking

[rappie]

Describe the desired feature

This would be helpful when ending shrinking prematurely or in cases where Echidna crashes while closing.

[arcz]

To give more context: we need to save the best shrink so far and override the previous, but this required some changes to the file names as we currently name them after the content hash. Otherwise, we produce thousands of files which are mostly useless.