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`.
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>`.
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`.
* 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.
