rename ch-grow to ch-image

Per discussion, we are going to rename ch-grow to ch-image because it now does a lot more than build images.

This issue is to:

Rename the script itself.
Change all the code and docs.
Provide a wrapper script ch-grow that says the old name is deprecated and will be deleted in 0.23. This script should print the warning, sleep 5, pass on everything to ch-image, and then print the warning again.

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