Nostr MMR implementation

Demo client using merkle mountain range (MMR) as an append only vector commitment for events (the same data structure used by opentimestamps in NIP-03) to enable fast verification (only a few events needed for proof of membership in the MMR).

This was briefly sketched here

In a Nostr event, we add tag MMR that includes the prev_event_id and the prev_mmr_root and the prev_event_pos.

    Mmr {
        prev_event_id: Hash,
        prev_mmr_root: Hash,
        prev_event_pos: i64,
    }

The tag is defined here

We forked extracted code from grin MMR (which is monolithic): https://github.com/Pencil-Yao/cloud-mmr/compare/master...nostronaut:cloud-mmr:master and replaced the crypto to bitcoin_hashes

Example of 8 chained events using this demo code:

<-event 0 <-event 1 <-event 2 <-event 3 <-event 4 <-event 5 <-event 6 <-event 7

The encoded MMR encoded in the MMR tag looks like this:

2            111
           /     \
1         11     110       1010
         /  \    / \      /    \
0       1   10 100 101  1000  1001  1011
event   e0  e1  e2  e3   e4    e5    e6    e7