[bdk_chain_redesign] Better names, comments and generic bounds

* Instead of implementing `ChainPosition` for `ObservedIn<BlockId>` to
  use `FullTxOut` methods (`is_spendable_at` and `is_mature`), we create
  alternative versions of those methods that require bounds with `Anchor`.
  This removes all `ObservedIn<A>: ChainPosition` bounds for methods of
  `IndexedTxGraph`.

* Various improvements to comments and names.

crates/chain/src/local_chain.rs

@@ -123,7 +123,7 @@ impl LocalChain {
 
     /// Updates [`LocalChain`] with an update [`LocalChain`].
     ///
-    /// This is equivilant to calling [`determine_changeset`] and [`apply_changeset`] in sequence.
+    /// This is equivalent to calling [`determine_changeset`] and [`apply_changeset`] in sequence.
     ///
     /// [`determine_changeset`]: Self::determine_changeset
     /// [`apply_changeset`]: Self::apply_changeset