[bdk_chain_redesign] Better names, comments and generic bounds

[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.