Add helper for getting claimable balance

It is annoying to have to match across all the enums of `Balance` to
just pull out the `claimable_amount_satoshis` value. This helper makes
it easier if you just want to amount.

lightning/src/chain/channelmonitor.rs

@@ -651,6 +651,40 @@ pub enum Balance {
 	},
 }
 
+impl Balance {
+	/// The amount claimable, in satoshis. This could be restricted by a timeout or potentially
+	/// claimed by our counterparty. See the variant documentation for more details.
+	///
+	/// On-chain fees required to claim the balance are not included in this amount.
+	pub fn claimable_amount_satoshis(&self) -> u64 {
+		match self {
+			Balance::ClaimableOnChannelClose {
+				claimable_amount_satoshis,
+			} => *claimable_amount_satoshis,
+			Balance::ClaimableAwaitingConfirmations {
+				claimable_amount_satoshis,
+				..
+			} => *claimable_amount_satoshis,
+			Balance::ContentiousClaimable {
+				claimable_amount_satoshis,
+				..
+			} => *claimable_amount_satoshis,
+			Balance::MaybeTimeoutClaimableHTLC {
+				claimable_amount_satoshis,
+				..
+			} => *claimable_amount_satoshis,
+			Balance::MaybePreimageClaimableHTLC {
+				claimable_amount_satoshis,
+				..
+			} => *claimable_amount_satoshis,
+			Balance::CounterpartyRevokedOutputClaimable {
+				claimable_amount_satoshis,
+				..
+			} => *claimable_amount_satoshis,
+		}
+	}
+}
+
 /// An HTLC which has been irrevocably resolved on-chain, and has reached ANTI_REORG_DELAY.
 #[derive(PartialEq, Eq)]
 struct IrrevocablyResolvedHTLC {