Add a parameter to bridgetree that sets a power of two defining a chunk size for bridge splits.

[nuttycom]

In the use case where we're doing a DAG-based traversal of notes, we need the two following things:

• The ability to retain a set of checkpoints at (configurably) 2^16 note boundaries such that they are not garbage-collected.
• An operation mark_at that, given a position p, can request an iterator full of leaves over a given range of positions, and replace the bridge containing p with a pair of bridges with the leaf at position p witnessed.

    pub fn mark_at<G, F, I>(&mut self, position: Position, get_context: F) -> Result<(), SplitError>
        where
            I: Iterator<Item = H>,
            F: FnOnce(Range<Position>) -> I

[daira]

Is this issue obsolete given the move to shardtree?