Fixes#251
TxBuilders are now not created directly but are created through the
wallet with `build_tx` and `build_fee_bump`.
The advantages of this realised in this commit are:
1. Normal tx creation and fee bumping use the code internally. The only
difference between normal tx and fee bump is how the builder is created.
2. The TxBuilder now has a refernce to the wallet and can therefore
lookup things as methods are called on it. `add_utxo` now uses this to
look up UTXO deta when it is called (rather than having to do it and
possibly error later on).
To support these changes `get_utxo` and `get_descriptor_for_keychain`
public methods have been added to Wallet. I could have kept them
pub(crate) but they seem like fine APIs to have publicly.
Instead of having two traits, `Blockchain` and `OnlineBlockchain` that need
to be implemented by the user, only the relevant one (`OnlineBlockchain`, here
renamed to `Blockchain`) will need to be implemented, since we provide a
blanket implementation for the "marker" trait (previously `Blockchain`, here
renamed to `BlockchainMarker`).
Users of the library will probably never need to implement `BlockchainMarker`
by itself, since we expose the `OfflineBlockchain` type that already does
that and should be good for any "offline" wallet. Still, it's exposed since
they might need to import it to define types with generics.
