[chain_redesign] Remove old structures

Other changes:

* The `async-https` feature of `bdk_esplora` is no longer default.
* Rename `ObservedAs` to `ChainPosition`.
* Set temporary MSRV to 1.60.0 to compile all workspace members will all
  features.

crates/chain/src/keychain/txout_index.rs

@@ -166,7 +166,10 @@ impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
     ///
     /// This will panic if a different `descriptor` is introduced to the same `keychain`.
     pub fn add_keychain(&mut self, keychain: K, descriptor: Descriptor<DescriptorPublicKey>) {
-        let old_descriptor = &*self.keychains.entry(keychain).or_insert(descriptor.clone());
+        let old_descriptor = &*self
+            .keychains
+            .entry(keychain)
+            .or_insert_with(|| descriptor.clone());
         assert_eq!(
             &descriptor, old_descriptor,
             "keychain already contains a different descriptor"