Multi token standard

[ilblackdragon]

Contracts like Ref Finance or EVM contain many tokens internally.
Currently there is no API to surface even which tokens are inside, or transfer / get balances of them.

Either extending or adding a general standard that for example defines some API to query this information (like "<near_account_id>:<internal_token_id>" or first argument in the function to be <internal_token_id>) to allow all the tooling to work with these tokens is critical.

[ilblackdragon]

Other approach is we say that we have a fallback from accounts <token_id>.<account_id> to <account_id> if such actual account doesn't exist.

This approach requires that all subaccounts (at least at some level) live on the same shard.
Also it requires some API for contracts to handle this fallback, where it distinguishes current_account_id been this subaccount. For example, current_subaccount_id is empty if it's direct call and non empty when it's not.

This is also related to WASM in WASM, where wasm contracts can be named with subaccounts and can be called directly from other contracts as if they are native contracts via promises.