pls add 'remove fully' option to remove() function

[fiag]

According the npm/cacache rm.entry API

By default, this appends a new entry to the index with an integrity of null. If opts.removeFully is set to true then the index file itself will be physically deleted rather than appending a null

thanks a lot!

[zkat]

doing this would mean that the index would no longer be atomic. That's why deletions are done through appending null.

the JS version of cacache has a verify() method that is expected to be called in a "locked" environment when nothing else is running. I'd like to have that in cacache-rs too.

[fiag]

After read verify() method, seem that this method does not delete index files. And the index with an integrity of null will be ignored in the index::ls() method.