Translate lending to the new syntax

[virgil-serbanuta]

Relevant for #117

We would need to:

• Use the new attributes for the contract, endpoints and storage (#[ethereum::contract], #[endpoint(endpointName)], #[storage("storage_name")])
• Use the new data types (Int256 for amounts, Address for addresses, Storage for the storage object) and remove the old ones (e.g. BigUint, StorageMapper).
• Use the new Hooks trait for blockchain functionality instead of the old blockchain/send/whatever functions.
• Use "Bytes" wherever we need bytes.