Implement updates that returns position in block and merkle proofs

[LLFourn]

We have built the internals to allow us to include position in block and even a full merkle proof for the transactoin in ChainIndex. Let's do it and show it in an example.

cc @evanlinjin a decent candidate for this would be creating a ChainGraph update from blocks fetched from rpc. It could be done for electrum too (see #77) because iirc getting the position is the same as getting the merkle root. This should be a different call though because you don't always want this.

Another thing worth considering is that including the position in block makes the ChainIndex a total order so we may need to extend that trait to tell us whether the index is a total order. This will allow us to make sure no two chain indexes are the same if it is a total order (i.e. no transactions are at the same height in the same block).