Introduce monadic AmountOpResult

We would like to return an error when doing math ops on amount types.
We cannot however use the stdlib `Result` or `Option` because we want to
implement ops on the result type.

Add an `AmountOpResult` type. Return this type from all math operations
on `Amount` and `SignedAmount`.

Implement `core::iter::Sum` for the new type to allow summing iterators
of amounts - somewhat ugly to use, see tests for example usage.

bitcoin/examples/taproot-psbt.rs

@@ -412,7 +412,7 @@ impl BenefactorWallet {
             taproot_spend_info.internal_key(),
             taproot_spend_info.merkle_root(),
         );
-        let value = input_utxo.amount - ABSOLUTE_FEES;
+        let value = (input_utxo.amount - ABSOLUTE_FEES).unwrap();
 
         // Spend a normal BIP86-like output as an input in our inheritance funding transaction
         let tx = generate_bip86_key_spend_tx(
@@ -476,7 +476,7 @@ impl BenefactorWallet {
             let mut psbt = self.next_psbt.clone().expect("should have next_psbt");
             let input = &mut psbt.inputs[0];
             let input_value = input.witness_utxo.as_ref().unwrap().value;
-            let output_value = input_value - ABSOLUTE_FEES;
+            let output_value = (input_value - ABSOLUTE_FEES).into_result()?;
 
             // We use some other derivation path in this example for our inheritance protocol. The important thing is to ensure
             // that we use an unhardened path so we can make use of xpubs.
@@ -649,7 +649,7 @@ impl BeneficiaryWallet {
         psbt.unsigned_tx.lock_time = lock_time;
         psbt.unsigned_tx.output = vec![TxOut {
             script_pubkey: to_address.script_pubkey(),
-            value: input_value - ABSOLUTE_FEES,
+            value: (input_value - ABSOLUTE_FEES).unwrap(),
         }];
         psbt.outputs = vec![Output::default()];
         let unsigned_tx = psbt.unsigned_tx.clone();

bitcoin/src/blockdata/transaction.rs

@@ -1673,7 +1673,7 @@ mod tests {
 
         // 10 sat/kwu * (204wu + BASE_WEIGHT) = 4 sats
         let expected_fee = "4 sats".parse::<SignedAmount>().unwrap();
-        let expected_effective_value = value.to_signed() - expected_fee;
+        let expected_effective_value = (value.to_signed() - expected_fee).unwrap();
         assert_eq!(effective_value, expected_effective_value);
     }
 

bitcoin/src/psbt/mod.rs

@@ -2249,7 +2249,7 @@ mod tests {
         };
         assert_eq!(
             t.fee().expect("fee calculation"),
-            prev_output_val - (output_0_val + output_1_val)
+            (prev_output_val - (output_0_val + output_1_val).unwrap()).unwrap()
         );
         // no previous output
         let mut t2 = t.clone();

units/src/amount/mod.rs

@@ -6,6 +6,7 @@
 //! We refer to the documentation on the types for more information.
 
 mod error;
+mod result;
 #[cfg(feature = "serde")]
 pub mod serde;
 
@@ -34,6 +35,7 @@ pub use self::{
         OutOfRangeError, ParseAmountError, ParseDenominationError, ParseError,
         PossiblyConfusingDenominationError, TooPreciseError, UnknownDenominationError,
     },
+    result::{NumOpError, NumOpResult},
     signed::SignedAmount,
     unsigned::Amount,
 };