descriptors: add method to get max tx weight

Errors in the weight calculation could be hidden when converting
to vbytes. This separate method will make it easier to spot any
such errors.

liana/src/descriptors/mod.rs

@@ -545,20 +545,25 @@ impl LianaDescriptor {
         Ok(self.prune_bip32_derivs(psbt, path_info))
     }
 
-    /// Maximum possible size in vbytes of an unsigned transaction, `tx`,
+    /// Maximum possible weight in weight units of an unsigned transaction, `tx`,
     /// after satisfaction, assuming all inputs of `tx` are from this
     /// descriptor.
-    pub fn unsigned_tx_max_vbytes(&self, tx: &bitcoin::Transaction, use_primary_path: bool) -> u64 {
-        let witness_factor: u64 = WITNESS_SCALE_FACTOR.try_into().unwrap();
+    fn unsigned_tx_max_weight(&self, tx: &bitcoin::Transaction, use_primary_path: bool) -> u64 {
         let num_inputs: u64 = tx.input.len().try_into().unwrap();
         let max_sat_weight: u64 = self.max_sat_weight(use_primary_path).try_into().unwrap();
         // Add weights together before converting to vbytes to avoid rounding up multiple times.
-        let tx_wu = tx
-            .weight()
+        tx.weight()
             .to_wu()
             .checked_add(max_sat_weight.checked_mul(num_inputs).unwrap())
-            .unwrap();
-        tx_wu
+            .unwrap()
+    }
+
+    /// Maximum possible size in vbytes of an unsigned transaction, `tx`,
+    /// after satisfaction, assuming all inputs of `tx` are from this
+    /// descriptor.
+    pub fn unsigned_tx_max_vbytes(&self, tx: &bitcoin::Transaction, use_primary_path: bool) -> u64 {
+        let witness_factor: u64 = WITNESS_SCALE_FACTOR.try_into().unwrap();
+        self.unsigned_tx_max_weight(tx, use_primary_path)
             .checked_add(witness_factor.checked_sub(1).unwrap())
             .unwrap()
             .checked_div(witness_factor)