[revive]: exposing `to_account_id` to smart contracts

[peterwht]

As pallet-revive uses AccountId20, ink! (and other languages) are unable to properly represent Polkadot-native accounts in the smart contract. Currently, there is not a way for a smart contract to access the mapping from AccountId20 -> AccountId32.

By exposing the mapping to_account_id, ink! will be able to represent and use the Polkadot native accounts that call the contract.

One example for a use-case is if an ink! contract wants to calculate the computed origin of an account on a different parachain, before sending an XCM.

Another example is if an ink! contract wants to send an XCM (locally, or cross-chain) that uses some pallet with input based on the calling user, however, the pallet requires an AccountId32.