keychain::ChangeSet includes the descriptor

keychain::ChangeSet includes the descriptor

- The KeychainTxOutIndex's internal SpkIterator now uses DescriptorId
  instead of K. The DescriptorId -> K translation is made at the
  KeychainTxOutIndex level.
- The keychain::Changeset is now a struct, which includes a map for last
  revealed indexes, and one for newly added keychains and their
  descriptor.

API changes in bdk:
- Wallet::keychains returns a `impl Iterator` instead of `BTreeMap`
- Wallet::load doesn't take descriptors anymore, since they're stored in
  the db
- Wallet::new_or_load checks if the loaded descriptor from db is the
  same as the provided one

API changes in bdk_chain:
- `ChangeSet` is now a struct, which includes a map for last revealed
  indexes, and one for keychains and descriptors.
- `KeychainTxOutIndex::inner` returns a `SpkIterator<(DescriptorId, u32)>`
- `KeychainTxOutIndex::outpoints` returns a `impl Iterator` instead of `&BTreeSet`
- `KeychainTxOutIndex::keychains` returns a `impl Iterator` instead of
  `&BTreeMap`
- `KeychainTxOutIndex::txouts` doesn't return a ExactSizeIterator
  anymore
- `KeychainTxOutIndex::unbounded_spk_iter` returns an `Option`
- `KeychainTxOutIndex::next_index` returns an `Option`
- `KeychainTxOutIndex::last_revealed_indices` returns a `BTreeMap`
  instead of `&BTreeMap`
- `KeychainTxOutIndex::reveal_to_target` returns an `Option`
- `KeychainTxOutIndex::reveal_next_spk` returns an `Option`
- `KeychainTxOutIndex::next_unused_spk` returns an `Option`
- `KeychainTxOutIndex::add_keychain` has been renamed to
  `KeychainTxOutIndex::insert_descriptor`, and now it returns a
  ChangeSet