Rather than returning an OpID for every mutation, we now return an
`Option<ObjId>`. This is `Some` only when a `make*` operation was
applied. This `ObjID` is an opaque type which can be used with any
document.
Repeatedly setting the same value for a particular (obj, key)
combination now no longer generates an operation. To allow this we
modify the return value of `automerge::set` so that it may return an
`Option<OpId>` instead of an `OpId`.
Signed-off-by: Alex Good <alex@memoryandthought.me>
