Replace set_single_recipient with drain_to

What set_single_recipient does turns out to be useful with multiple
recipients.
Effectively, set_single_recipient was simply creating a change
output that was arbitrarily required to be the only output.
But what if you want to send excess funds to one address but still have
additional recipients who receive a fixed value?
Generalizing this to `drain_to` simplifies the logic and removes several
error cases while also allowing new use cases.

"maintain_single_recipient" is also replaced with "allow_shrinking"
which has more general semantics.

src/error.rs

@@ -24,10 +24,6 @@ pub enum Error {
     Generic(String),
     /// This error is thrown when trying to convert Bare and Public key script to address
     ScriptDoesntHaveAddressForm,
-    /// Found multiple outputs when `single_recipient` option has been specified
-    SingleRecipientMultipleOutputs,
-    /// `single_recipient` option is selected but neither `drain_wallet` nor `manually_selected_only` are
-    SingleRecipientNoInputs,
     /// Cannot build a tx without recipients
     NoRecipients,
     /// `manually_selected_only` option is selected but no utxo has been passed