Further enhancements to `Update` and `ChangeSet` structures

[evanlinjin]

Describe the enhancement

As suggested by @LLFourn:

1. LocalUpdate should be moved to bdk Wallet.
2. We shouldn't use LocalUpdate in electrum API (it doesn't need to be there IMO).
3. Delete LocalChangeSet or move it to bdk Wallet. We should implement Append for tuples (A,B) where A: Append, B:Append and use this as our changeset in the examples instead of LocalChangeSet. i.e. we can just go type ChangeSet = (local_chain::ChangeSet, indexed_tx_graph::ChangeSet<...>)

[evanlinjin]

We also need to remove preview_.. methods from TxGraph since it is a monotone structure now.

[evanlinjin]

Something else I realized, is the ForEachTxOut trait strictly needed still?

[LLFourn]

We also need to remove preview_.. methods from TxGraph since it is a monotone structure now.

This is done in #1065 I think.

[vladimirfomene]

Hi @evanlinjin ! I will like to take this up.

[evanlinjin]

@vladimirfomene Thanks for taking this on. Go hard!