#38: New update_one method

[kirillt]

New update_one method enables library users to update a single entry in the index.

This is useful for scenarios where we performed some action on a resource and want to quickly adjust app model with the index. More details in the issue:

• Lightweight, single-resource update method #38

The usage in an app should be like this:

1. App has the index. It is allowed to be slightly outdated.
2. User deletes or edits a resource. New resource by the same path has different content.
3. Full updating using update_all would spend some time and user could be annoyed by necessity to wait before they can move to next resource in their selection. That's why we call update_one and pass path of the edited resource.
4. We receive update from the index and handle it uniformly to any external updates.
5. Since we used this update_one method, the index still can be outdated, same as in step 1. That's fine because the app model and the index are in sync.
6. When the user is done with their selection, we can call update_all and catch up with external updates to the filesystem.

[kirillt]

Double-check that Pdfium works. Nightly was introduced when we were fixing it.