Merge bitcoindevkit/bdk#1390: Make Wallet require a change descriptor

8bc3d35f6c83d078551c4bc57f72f13324865486 fix(wallet): `LoadError::MissingDescriptor` includes the missing KeychainKind (valued mammal)
412dee1f5b027af7800ee155717d47f4ab4b60e0 ref(wallet)!: Make `Wallet::public_descriptor` infallible (valued mammal)
c2513e1090374d7871a8623845bd10757e6ab0b3 test(wallet): Clarify docs for get_funded_wallet (valued mammal)
9d954cf7d25d43c77115cd931407005a09365389 refactor(wallet)!: Make Wallet require a change descriptor (valued mammal)

Pull request description:

  All `Wallet` constructors are modified to require a change descriptor, where previously it was optional. Additionally we enforce uniqueness of the change descriptor to avoid ambiguity when deriving scripts and ensure the wallet will always have two distinct keystores.

  Notable changes

  * Add error `DescriptorError::ExternalAndInternalAreTheSame`
  * Remove error `CreateTxError::ChangePolicyDescriptor`
  * No longer rely on `map_keychain`

  fixes #1383

  ### Notes to the reviewers

  ### Changelog notice

  Changed:

  Constructing a Wallet now requires two distinct descriptors.

  ### 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

ACKs for top commit:
  notmandatory:
    re-ACK 8bc3d35f6c83d078551c4bc57f72f13324865486

Tree-SHA512: f0621deb75d8e1e484b18b40d850f64e26314e39c4778f56c627763ddbffd376288bf6f0f37b61ba2ba744c7083683497d2dfef42bc4ef7d3ed7b374a54d813a