vaults: make recovery transaction explicit

Instead of implicitly detecting whether or not an OP_VAULT/OP_UNVAULT
spend is a recovery spend by scanning outputs for matching
scriptPubKeys, explicitly indicate recoveries by requiring a witness
stack element that is either -1 in the case of no recovery OR
corresponds to an output index that is the recovery output.

Author: jamesob

bip-vaults.mediawiki

@@ -279,32 +279,26 @@ where
 ** If <code><recovery-params></code> is less than 32 bytes, script execution when spending this output MUST fail and terminate immediately.
 ** Because the recovery scriptPubKey is committed to with a hash, witness version upgradeability is preserved.
 
-==== Check for recovery ====
+==== Witness stack ====
 
 After the witness program is parsed, it must be determined whether this input
-is being spent towards a recovery. If an output in the spending transaction is
-found whose scriptPubKey hashes to the recovery sPK hash (the
-first component of <code><recovery-params></code>), the interpreter will
-evaluate for recovery. Otherwise, the interpreter will evaluate assuming a withdrawal
-is being triggered.
+is being spent towards a recovery. 
 
-In pseudocode:
+Witness stack shown top to bottom:
 
-<source lang="python">
-is_recovery = False
-recovery_out: Optional[CTxOut] = None
-
-for out in spending_tx.vout:
-    if tagged_hash("VaultRecoverySPK", out.scriptPubKey) == recovery_sPK_hash:
-        is_recovery = True
-        recovery_out = out
-
-if is_recovery:
-    eval_for_recovery(recovery_out)
-else:
-    eval_for_withdrawal_trigger()
+<source>
+<recovery-vout-idx>
+[other potential witness stack items ...]
 </source>
 
+where
+
+* <code><recovery-vout-idx></code> is an integer indicating which output, if any, is a recovery output.
+** If this value cannot be decoded as a CScriptNum and cast to an integer, script execution MUST fail and terminate immediately.
+** If this value is less than -1, script execution MUST fail and terminate immediately.
+** If this value is greater than or equal to 0, this spend is a recovery transaction and this value denotes the recovery output that corresponds to this vault input.
+* The parse of the other stack items depends on whether or not this is a recovery spend.
+
 ==== <code>OP_VAULT</code> evaluation for recovery spend ====
 
 * If the recovery output does not have an <code>nValue</code> greater than this input's amount, the script MUST fail and terminate immediately.