Allow compaction from lib

[rubend056]

Is there any way the compaction interface can be reformatted so its exported from lib.

This is extremely useful as it allows compaction to be done at regular intervals from a downstream crate executable, instead of having to manually run the sonnerie command line tool.

I will try to give it a go on a pull request, but I remember looking at it before and it not being trivial.

[njaard]

Hi @rubend056 , in fact the only code you really need is to read and write the database again, https://github.com/njaard/sonnerie/blob/master/src/main.rs#L490 and then the _purge_compacted_files

It's not part of the API because I just didn't know what it would look like.

[rubend056]

Sorry for the delayed response, I'll make a commit to my fork and a pull request today :) let me know of any issues you can spot.