Use TX IDs for Bitcoin Eventualities

They're a bit more binding, smaller, provided by the Rust bitcoin library,
sane, and we don't have to worry about malleability since all of our inputs are
SegWit.

coins/bitcoin/src/wallet/send.rs

@@ -13,6 +13,7 @@ use k256::{elliptic_curve::sec1::ToEncodedPoint, Scalar};
 use frost::{curve::Secp256k1, Participant, ThresholdKeys, FrostError, sign::*};
 
 use bitcoin::{
+  hashes::Hash,
   sighash::{TapSighashType, SighashCache, Prevouts},
   absolute::LockTime,
   script::{PushBytesBuf, ScriptBuf},
@@ -245,6 +246,13 @@ impl SignableTransaction {
     })
   }
 
+  /// Returns the TX ID of the transaction this will create.
+  pub fn txid(&self) -> [u8; 32] {
+    let mut res = self.tx.txid().to_byte_array();
+    res.reverse();
+    res
+  }
+
   /// Returns the outputs this transaction will create.
   pub fn outputs(&self) -> &[TxOut] {
     &self.tx.output