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`
