indexing text document and cursor tracking

[munhitsu]

To really use Automerge for Text editor one needs to keep cursor position updated while remote changes are arriving. One way of doing it is to expose 2 functions to translate from cursor Int position to ObjectID and ObjectID to a position.

Such 2 APIs will also allow to index chunks of the live documents.

The other way is to manually process remote changes and apply them to the cursor position, but it's duplicating the rust library code.

I might have a bad day but I can't figure out how to do the option 1 with the current library version.