Introduce an iterator type for script_pubkey_lens

Currently `script_pubkey_lens` returns a generic `Iterator` using `impl`
syntax. This syntax is not supported in traits and we want to move the
function to the soon-to-be-added `TransactionExt` trait.

Add a struct to hold the iterator returned by `Map`, this is ugly but
its the least ugly thing I could come up with.

bitcoin/src/blockdata/transaction.rs

@@ -501,8 +501,8 @@ impl Transaction {
     /// 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> + '_ {
+    pub fn script_pubkey_lens(&self) -> TxOutToScriptPubkeyLengthIter {
-        self.output.iter().map(|txout| txout.script_pubkey.len())
+        TxOutToScriptPubkeyLengthIter { inner: self.output.iter() }
     }
 
     /// Counts the total number of sigops.
@@ -545,6 +545,18 @@ impl Transaction {
     }
 }
 
+/// Iterates over transaction outputs and for each output yields the length of the scriptPubkey.
+// This exists to hardcode the type of the closure crated by `map`.
+pub struct TxOutToScriptPubkeyLengthIter<'a> {
+    inner: core::slice::Iter<'a, TxOut>,
+}
+
+impl Iterator for TxOutToScriptPubkeyLengthIter<'_> {
+    type Item = usize;
+
+    fn next(&mut self) -> Option<usize> { self.inner.next().map(|txout| txout.script_pubkey.len()) }
+}
+
 impl Transaction {
     /// Gets the sigop count.
     ///