* `Additions` now implements `Append` and uses `Append` to implement
`append()`.
* `append()` logic enforces that `last_seen` values should only
increase.
* Test written for `append()` with `last_seen` behaviour.
The `ConfirmationHeight` trait has been removed in favour of a second
method on the `Anchor` trait: `confirmation_height_upper_bound()`.
Methods `try_balance_at()` and `balance_at()` of `IndexedTxGraph` have
been removed as they do not provide additional functionality.
`IndexedTxGraph::insert_relevant_txs` now uses two loops, the first loop
indexes all transactions first. This is done as some indexes require
ancestor transactions to be indexed first and we cannot guarantee that
the input transactions are in topological order.
* Instead of implementing `ChainPosition` for `ObservedIn<BlockId>` to
use `FullTxOut` methods (`is_spendable_at` and `is_mature`), we create
alternative versions of those methods that require bounds with `Anchor`.
This removes all `ObservedIn<A>: ChainPosition` bounds for methods of
`IndexedTxGraph`.
* Various improvements to comments and names.
Methods of old structures that return transaction(s) no longer return
`TxNode`, but `Transaction` as done previously.
`TxInGraph` is renamed to `TxNode`, while the internal `TxNode` is
renamed to `TxNodeInternal`.
* Introduce `GraphedTx` struct to access transaction data of graphed
transactions.
* Ability to insert/access anchors and "seen at" values for graphed
transactions.
* `Additions` now records changes to anchors and last_seen_at.
As mentioned by @LLFourn:
1. We have a "sparse chain" from which there is a subset of txids M that are missing from graph.
2. There is also another subset C that are in the graph but their positions have changed.
3. We used the Cow to avoid copying/duplicating in memory transactions in subset C and M
Instead in inflate_update we could remove transactions in subset M and just clone data in subset C (which is usually tiny).
