Merge bitcoindevkit/bdk#1203: Include the descriptor in `keychain::Changeset`

86711d4f46f467c651238ad3804fdbe1d22a8600 doc(chain): add section for non-recommended K to descriptor assignments (Daniela Brozzoni)
de53d721913537f56281a134270eafd356f908ad test: Only the highest ord keychain is returned (Daniela Brozzoni)
9d8023bf56a693f1cb2ba340ed024c654307c069 fix(chain): introduce keychain-variant-ranking to `KeychainTxOutIndex` (志宇)
6c8748124fd40e0fee37f78ca30457441b13fbcb chore(chain): move `use` in `indexed_tx_graph.rs` so clippy is happy (志宇)
537aa03ae0f8bec4dc799d33738e9bb7977bdac1 chore(chain): update test so clippy does not complain (志宇)
ed117de7a5b1756482b2e6487855b80e97c597ba test(chain): applying changesets one-by-one vs aggregate should be same (志宇)
6a3fb849e86e0bc21086519ae0201b95ddde5bf4 fix(chain): simplify `Append::append` impl for `keychain::ChangeSet` (志宇)
1d294b734dd6f4639075cba271e2b40f437f998f fix: Run tests only if the miniscript feature is.. ..enabled, enable it by default (Daniela Brozzoni)
0e3e136f6fa7215f6391dbcc1c4781262111ce64 doc(bdk): Add instructions for manually inserting... ...secret keys in the wallet in Wallet::load (Daniela Brozzoni)
76afccc555feff084867d6f9406e4e74bee938cc fix(wallet): add expected descriptors as signers after creating from wallet::ChangeSet (Steve Myers)
4f05441a00b921efd661da0dff94d9c28e38b70d keychain::ChangeSet includes the descriptor (Daniela Brozzoni)
8ff99f27dfe45643bf401409ca72429a9b812873 ref(chain): Define test descriptors, use them... ...everywhere (Daniela Brozzoni)
b9902936a0d17498eec8866233d89b2882d0af8f ref(chain): move `keychain::ChangeSet` into `txout_index.rs` (志宇)

Pull request description:

  Fixes #1101

  - Moves keychain::ChangeSet inside `keychain/txout_index.rs` as now the `ChangeSet` depends on miniscript
  - Slightly cleans up tests by introducing some constant descriptors
  - 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.

  ### Changelog notice

  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 `BTreeSet` instead of `&BTreeSet`
  - `KeychainTxOutIndex::keychains` returns a `impl Iterator` instead of
        `&BTreeMap`
  - `KeychainTxOutIndex::txouts` doesn't return a ExactSizeIterator anymore
  - `KeychainTxOutIndex::last_revealed_indices` returns a `BTreeMap`
        instead of `&BTreeMap`
  - `KeychainTxOutIndex::add_keychain` has been renamed to `KeychainTxOutIndex::insert_descriptor`, and now it returns a ChangeSet
  - `KeychainTxOutIndex::reveal_next_spk` returns Option
  - `KeychainTxOutIndex::next_unused_spk` returns Option
  - `KeychainTxOutIndex::unbounded_spk_iter` returns Option
  - `KeychainTxOutIndex::next_index` returns Option
  - `KeychainTxOutIndex::reveal_to_target` returns Option
  - `KeychainTxOutIndex::revealed_keychain_spks` returns Option
  - `KeychainTxOutIndex::unused_keychain_spks` returns Option
  - `KeychainTxOutIndex::last_revealed_index` returns Option
  - `KeychainTxOutIndex::keychain_outpoints` returns Option
  - `KeychainTxOutIndex::keychain_outpoints_in_range` returns Option
  - `KeychainTxOutIndex::last_used_index` returns None if the keychain has never been used, or if it doesn't exist

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature

ACKs for top commit:
  evanlinjin:
    ACK 86711d4f46f467c651238ad3804fdbe1d22a8600

Tree-SHA512: 4b1c9a31951f67b18037b7dd9837acbc35823f21de644ab833754b74d20f5373549f81e66965ecd3953ebf4f99644c9fd834812acfa65f9188950f1bda17ab60