* 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.
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`.
