The Major Wallet module refactoring.

This commits refactors the wallet to work with generic Keychains.

A default wallet is defined as `Wallet<D, KeychainKind>`. Which has the API
that will only work with a `KeychainKind` keychain type.

A generic <D, K> wallet impl is defined to add the multi descriptor wallet
API.

New APIs are added which are only applicable in multi descriptor
environment.

 - new_multi_descriptor() : create a multi descriptor wallet.
 - add_descriptor() : Adds a descriptor and marks it with a `K`.
 - get_address_by_keychain (): Get an address for a given keychain.
 - list_utxos_by_keychain (): does what it says.
 - list_transactions_by_keychain() : ''
 - get_balance_by_keychain() : Get a balance for one keychain. The unconfirmed
 balance is reported as trusted.
 - get_balance_for_keychains() : Get accumulated balance for a list of keychains.
 uses `should_trust` predicate to determine "untrusted" keychains.
 - create_tx_multi() : The multi descriptor create transaction API. It takes in a list of keychains to spend from. And a list of change keychains to
 generate change address from. For `change_keychains` is an iterator instead of a single keychain, because later we wanna add recipient address matching
 capability of the change address. Currently it does nothing.

 The default wallet APIs are changed to work over the multi-desc API as
 special cases. All other APIs remains same.

Co-authored-by: Vladimir Fomene <vladimirfomene@gmail.com>

Author: rajarshimaitra