* 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.

The chain oracle keeps track of the best chain, while the transaction
index indexes transaction data in relation to script pubkeys.

This commit also includes initial work on `IndexedTxGraph`.

Add methods to `TxGraph` and `IndexedTxGraph` that gets in-best-chain
data (such as transactions, txouts, unspent txouts).

Methods that list chain data have try and non-try versions. Both of
these versions now return an `Iterator`.

* Try versions return `Iterator<Item = Result>`.
* Non-try versions require the `ChainOracle` implementation to be
  `ChainOracle<Error = Infallible>`.

* Get mutable index from `IndexedChainGraph`.
* Also add `apply_additions` method to `TxIndex` trait.

…height

This is important as a `ChainOracle` implementation is updated
separately to an `IndexedTxGraph`.

This allows us to use the old API with minimal changes. `TxGraph`
methods have also been rearranged to allow for it.

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`.

* Rename `TxNode::last_seen` to `last_seen_unconfirmed` and improve docs
* Improve `try_get_chain_position` logic and tweak comments

Also made the `IndexedTxGraph::index` field public (`index()` and
`index_mut()` methods are no longer needed).

Introduce `chain_oracle::Cache` which is a cache for requests to the
chain oracle. `ChainOracle` has also been moved to the `chain_oracle`
module.

Introduce `get_tip_in_best_chain` method to the `ChainOracle` trait.
This allows for guaranteeing that chain state can be consistent across
operations with `IndexedTxGraph`.

* 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.

Before, we were using `core::ops::AddAsign` but it was not the most
appropriate.

This makes the API of `TxIndex` more consistent between scanning in data
and checking whether certain data is relevant.

The problem with the previous `ChainOracle` interface is that it had no
guarantee for consistency. For example, a block deemed to be part of the
"best chain" can be reorged out. So when `ChainOracle` is called
multiple times for an operation (such as getting the UTXO set), the
returned result may be inconsistent.

This PR changes `ChainOracle::is_block_in_chain` to take in another
input `static_block`, ensuring `block` is an ancestor of `static_block`.
Thus, if `static_block` is consistent across the operation, the result
will be consistent also.

`is_block_in_chain` now returns `Option<bool>`. The `None` case means
that the oracle implementation cannot determine whether block is an
ancestor of static block. `IndexedTxGraph::list_chain_txouts` handles
this case by checking child spends that are in chain, and if so, the
parent tx must be in chain too.

Remove the requirement that evicted blocks should have in-best-chain
counterparts in the update.

It is better to have this external to this structure.

`tx_graph.rs` is rearranged as well.

Previously, I have misunderstood the definition of anchor. If a tx is
anchored in a block, it does not necessarily mean it is confirmed in
that block. The tx can be confirmed in an ancestor block of the anchor
block.

With this new definition, we need a new trait `ConfirmationHeight` that
has one method `confirmation_height`. This trait can be used to extend
`Anchor` for those implementations that can give us the exact
conirmation height of a tx (which is useful in most cases).

Another change is to add another variant to the `ObservedAs` enum;
`ObservedAs::ConfirmedImplicit(A)`. If a tx does not have an anchor, but
another tx that spends it has an anchor that in in the best chain, we
can assume that tx is also in the best chain. The logic of
`TxGraph::try_get_chain_position` is also changed to reflect this.

Some methods from `IndexedTxGraph` have been moved to `TxGraph` as they
do not require the `Indexer`. Some `TxGraph` methods have been renamed
for clarity and consistency.

Also more docs are added.

`ObservedAs::ConfirmedImplicit` is incomplete, remove for now.

`local_chain::ChangeSet` does not need to be a single-element tuple
struct.

This is mostly copying over the relevant tests from `SparseChain`.
Changes are made to `local_chain::ChangeSet` to re-add the ability to
remove blocks.

The `HashSet` was used for iterating without duplicate items. However,
since `anchors` is a `BTreeSet`, heights are in order. So a single
variable tracking last height will be sufficient.

* Remove `chain_oracle::CacheBackend` for now as it is not used.
* `SparseChain` does not need to implement `ChainOracle`.
* Remove filter predicate for `list..` methods of `TxGraph` and
  `IndexedTxGraph` as this is premature optimisation.
* `Append` can be implemented for all `BTreeMap`s and `BTreeSet`s,
  instead of only `local_chain::ChangeSet`.

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.

Ensure `insert_relevant_txs` does not require transactions to be in
topological order.

Other changes: Rm `try_list_owned_txs` as it is useless

The `insert_relevant_txs` test has also been changed to used
`KeychainTxOutIndex` so that index additions can be checked
(`SpkTxOutIndex` has no additions).

Additionally, generic bounds of some `IndexedTxGraph` list methods have
been fixed.

…able`

Docs are updated to explain why `is_mature` and
`is_confirmed_and_spendable` may return false-negatives.

Instead of forcing all transactions inserted to use the same anchors, we
change the API to have unique anchors per transaction.

This allows for more flexibility in general. For example, use `Anchor`
implementations that contain the position in a block of a transaction.

* `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.

* `IndexedTxGraph::try_balance` should include "confirmed and spendable"
  into `confirmed` balance.
* `TxGraph::try_list_chain_unspents` filter logic should be reversed.

`SpkTxOutIndex` and `KeychainTxOutIndex` now both implement
`OwnedIndexer`.

These tests cover list_txout, list_utxo and balance methods.