Add `script_pubkey_lens` method

In some cases people construct the transaction with a dummy fee output
value before calculating the weight. A method to create the iterator
over `script_pubkey` lengths is useful in such cases.

bitcoin/src/blockdata/transaction.rs

@@ -1014,6 +1014,15 @@ impl Transaction {
     pub fn is_lock_time_enabled(&self) -> bool {
         self.input.iter().any(|i| i.enables_lock_time())
     }
+
+    /// Returns an iterator over lengths of `script_pubkey`s in the outputs.
+    ///
+    /// This is useful in combination with [`predict_weight`] if you have the transaction already
+    /// constructed with a dummy value in the fee output which you'll adjust after calculating the
+    /// weight.
+    pub fn script_pubkey_lens(&self) -> impl Iterator<Item = usize> + '_ {
+        self.output.iter().map(|txout| txout.script_pubkey.len())
+    }
 }
 
 impl_consensus_encoding!(TxOut, value, script_pubkey);
@@ -1177,6 +1186,10 @@ impl From<&Transaction> for Wtxid {
 /// preceding compact size. The lenght of preceding compact size is computed and added inside the
 /// function for convenience.
 ///
+/// If you  have the transaction already constructed (except for signatures) with a dummy value for
+/// fee output you can use the return value of [`Transaction::script_pubkey_lens`] method directly
+/// as the second argument.
+///
 /// # Usage
 ///
 /// When signing a transaction one doesn't know the signature before knowing the transaction fee and