This allows for more robust error handling as we can now propagate errors up to the top level.

Specifically the Append type which takes a Result and returns a Result.

pub type Append<K, B> = dyn for<'a> Fn(
	&'a mut Context<'_, K, B>,
	Result<(Transaction, TxKernel, BlindSum), Error>,
) -> Result<(Transaction, TxKernel, BlindSum), Error>;

Also replaced with_fee() and with_lock_height() combinators with simply passing in kernel features to build::transaction() and build::partial_transaction(). It was confusing (and fragile) setting fee and lock_height individually via combinators. We now explicitly specify the features for the kernel of the tx being built.

let tx = transaction(
    KernelFeatures::Plain { fee: 4 },
    vec![
        input(6, key_id1),
        output(2, key_id2),
    ],
    &keychain,
    &builder,
)

NOTE: Wallet uses build::transaction() and build::partial_transaction() so we need to be careful merging this to make sure the wallet project is updated to reflect this.