Initial implementation of cursors (perf branch version)

[ept]

This PR is the same as #306, but based on the performance branch rather than main. It provides a first attempt at an API for cursors.

A cursor is an object of the form {objectId: 'string', elemId: 'string'} and it can be stored anywhere: inside an Automerge document or somewhere external. To get the cursor for a particular index of a text object, use:

const cursor = doc.text.getCursorAt(index)

The cursor identifies a particular character in the text; the position of that character may change as text is inserted or deleted before it. To get the current index of a previously created cursor, use:

const index = Automerge.getCursorIndex(doc, cursor, true)

The third argument controls what to do if the character referenced by the cursor has been deleted. If false and the character has been deleted, getCursorIndex returns -1. If the argument is true and the character has been deleted, getCursorIndex returns the index of the closest preceding non-deleted character, or -1 if there is no such preceding character.

I am not super happy with this approach, but putting up the PR nevertheless so that folks can play with it.

[ept]

I think we have come to the conclusion that the approach to cursors in #313 is preferable to this one, since it keeps open the option of having frontend and backend on separate threads. I will therefore close this PR.