Merge #974: Prepare Plan::update_psbt_input for migration to rust-psbt

0183420 chore: remove outdated TODO (nymius)
69be31b feat: expose `Plan.descriptor` field (nymius)

Pull request description:

  This is the simplest way I found to use the public `Plan` API in the `Plan::update_psbt_input` method so It can later be migrated to the `rust-psbt` project.
  
  **Notes to the reviewers**:
  - There is any reason to not expose the `descriptor` and `template` fields directly?
  - There is a **TODO** in the code, but I'm lacking the context of it. It is something that should be fixed or a leftover?


ACKs for top commit:
  apoelstra:
    ACK 0183420; successfully ran local tests


Tree-SHA512: a6164d1cfedfa69de9775273f11aa653805a2b10815b550a4305e89c117b24b4c9a722f70fd85a54e442173ae61a3fca38714bf7b4624cb06569739e4ea48ae8

Author: apoelstra

src/plan.rs

@@ -217,8 +217,8 @@ pub struct Plan<Pk: MiniscriptKey> {
     pub absolute_timelock: Option<absolute::LockTime>,
     /// The relative timelock this plan uses
     pub relative_timelock: Option<relative::LockTime>,
-
-    pub(crate) descriptor: Descriptor<Pk>,
+    /// The target descriptor for this plan
+    pub descriptor: Descriptor<Pk>,
 }
 
 impl<Pk: MiniscriptKey + ToPublicKey> Plan<Pk> {
@@ -363,8 +363,6 @@ impl Plan<DefiniteDescriptorKey> {
                     data
                 });
 
-            // TODO: TapTree. we need to re-traverse the tree to build it, sigh
-
             let leaf_hash = match data.spend_type {
                 Some(SpendType::KeySpend { internal_key }) => {
                     input.tap_internal_key = Some(internal_key);