We plan to record `Descriptor` additions into persistence. Hence, we
need to add `Descriptor`s to the changeset. This depends on
`miniscript`. Moving this into `txout_index.rs` makes sense as this is
consistent with all the other files. The only reason why this wasn't
this way before, is because the changeset didn't need miniscript.

Co-Authored-By: Daniela Brozzoni <danielabrozzoni@protonmail.com>

...everywhere

- 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

…wallet::ChangeSet

...secret keys in the wallet in Wallet::load

..enabled, enable it by default

We only need to loop though entries of `other`. The logic before was
wasteful because we were also looping though all entries of `self` even
if we do not need to modify the `self` entry.

This fixes the bug with changesets not being monotone. Previously, the
result of applying changesets individually v.s. applying the aggregate
of changesets may result in different `KeychainTxOutIndex` states.

The nature of the changeset allows different keychain types to share the
same descriptor. However, the previous design did not take this into
account. To do this properly, we should keep track of all keychains
currently associated with a given descriptor. However, the API only
allows returning one keychain per spk/txout/outpoint (which is a good
API).

Therefore, we rank keychain variants by `Ord`. Earlier keychain variants
have a higher rank, and the first keychain will be returned.