[plan] Make placeholders satisfiable

Each satisfiable placeholder now has an `Option<_>` where the data
needed to satisfy it can be inserted. This allows you satisfy the
witness without using a `Satisfier` impl. I think it also makes for a
simpler simpler API and internals. It probably needs some more work to
take advantage of it.

Author: LLFourn

src/descriptor/bare.rs

@@ -311,7 +311,7 @@ impl Pkh<DefiniteDescriptorKey> {
     {
         if provider.lookup_ecdsa_sig(&self.pk) {
             let stack = vec![
-                Placeholder::EcdsaSigPk(self.pk.clone()),
+                Placeholder::EcdsaSigPk(self.pk.clone(), None),
                 Placeholder::Pubkey(self.pk.clone(), BareCtx::pk_len(&self.pk)),
             ];
             Some(Plan {

src/descriptor/segwitv0.rs

@@ -430,7 +430,7 @@ impl Wpkh<DefiniteDescriptorKey> {
     {
         if provider.lookup_ecdsa_sig(&self.pk) {
             let stack = vec![
-                Placeholder::EcdsaSigPk(self.pk.clone()),
+                Placeholder::EcdsaSigPk(self.pk.clone(), None),
                 Placeholder::Pubkey(self.pk.clone(), Segwitv0::pk_len(&self.pk)),
             ];
             Some(Plan {

src/descriptor/tr.rs

@@ -319,8 +319,9 @@ impl<Pk: MiniscriptKey + ToPublicKey> Tr<Pk> {
     {
         let satisfaction =
             best_tap_spend(self, satisfier, false /* allow_mall */).map_stack(|stack| {
-                WitnessTemplate::from_placeholder_stack(stack)
-                    .try_completing(satisfier)
+                let mut wt = WitnessTemplate::from_placeholder_stack(stack);
+                wt.satisfy(satisfier);
+                wt.try_completing()
                     .expect("the same satisfier should manage to complete the template")
             });
         if let Witness::Stack(stack) = satisfaction.stack {
@@ -339,8 +340,9 @@ impl<Pk: MiniscriptKey + ToPublicKey> Tr<Pk> {
     {
         let satisfaction =
             best_tap_spend(self, satisfier, true /* allow_mall */).map_stack(|stack| {
-                WitnessTemplate::from_placeholder_stack(stack)
-                    .try_completing(satisfier)
+                let mut wt = WitnessTemplate::from_placeholder_stack(stack);
+                wt.satisfy(satisfier);
+                wt.try_completing()
                     .expect("the same satisfier should manage to complete the template")
             });
         if let Witness::Stack(stack) = satisfaction.stack {
@@ -661,6 +663,7 @@ where
             stack: Witness::Stack(vec![Placeholder::SchnorrSig(
                 desc.internal_key.clone(),
                 None,
+                None,
             )]),
             has_sig: true,
             absolute_timelock: None,