Add new utility methods on Transaction type

This PR adds the txid(), is_coin_base(), is_explicitly_rbf(),
and is_lock_time_enabled() methods.
Fixes #303

bdk-ffi/src/bdk.udl

@@ -247,13 +247,21 @@ interface Transaction {
   [Throws=BdkError]
   constructor(sequence<u8> transaction_bytes);
 
-  sequence<u8> serialize();
+  string txid();
 
   u64 weight();
 
   u64 size();
 
   u64 vsize();
+
+  sequence<u8> serialize();
+
+  boolean is_coin_base();
+
+  boolean is_explicitly_rbf();
+
+  boolean is_lock_time_enabled();
 };
 
 interface PartiallySignedTransaction {

bdk-ffi/src/lib.rs

@@ -251,8 +251,8 @@ impl Transaction {
         Ok(Transaction { internal: tx })
     }
 
-    fn serialize(&self) -> Vec<u8> {
-        self.internal.serialize()
+    fn txid(&self) -> String {
+        self.internal.txid().to_string()
     }
 
     fn weight(&self) -> u64 {
@@ -266,6 +266,22 @@ impl Transaction {
     fn vsize(&self) -> u64 {
         self.internal.vsize() as u64
     }
+
+    fn serialize(&self) -> Vec<u8> {
+        self.internal.serialize()
+    }
+
+    fn is_coin_base(&self) -> bool {
+        self.internal.is_coin_base()
+    }
+
+    fn is_explicitly_rbf(&self) -> bool {
+        self.internal.is_explicitly_rbf()
+    }
+
+    fn is_lock_time_enabled(&self) -> bool {
+        self.internal.is_lock_time_enabled()
+    }
 }
 
 /// A Bitcoin address.