Add tmp module around TxOut impl block

The two `TxOut` fields are public and there are no construtors or
getters to move, only the associated const `NULL`.

Add a tmp module around the big impl block so we can trick the formatter
into indenting before we add the extension trait.

bitcoin/src/blockdata/transaction.rs

@@ -354,6 +354,8 @@ impl TxOut {
         TxOut { value: Amount::from_sat(0xffffffffffffffff), script_pubkey: ScriptBuf::new() };
 }
 
+mod tmp {
+use super::*;
 impl TxOut {
     /// The weight of this output.
     ///
@@ -403,6 +405,7 @@ impl TxOut {
         TxOut { value: script_pubkey.minimal_non_dust_custom(dust_relay_fee), script_pubkey }
     }
 }
+}
 
 #[cfg(feature = "arbitrary")]
 impl<'a> Arbitrary<'a> for TxOut {