wallet - token support

[ixje]

from #80 (comment)

    def token_add(self, token_hash: UInt160) -> bool:
        # true if added, false if already exists

    def token_delete(self, token_hash: UInt160) -> bool:
        # true if deleted, false if failed if not found

    def token_delete_by_name(self, token_name: str) -> bool:
        # true if deleted, false if e.g. not found

My current view is that a Wallet is merely as a container holding Accounts. Therefore adding these functions to account means an Account is associated with 0 or more tokens. There can be helpers for Wallet to aggregate balances of a certain token in all available accounts, but that is beyond the scope. The extra field of the Account can be used for storing the token hashes when exporting.

[ixje]

The alternative is to add these functions to Wallet. The upside is that from a wallet you do a single import/add/delete. The downside is you're forced to use a Wallet and it has to search all accounts for balance every time, even though you might know certain accounts will never have any.

[luc10921]

How should the token_delete_by_name(self, token_name: str) method be implemented? Currently, the only way to add a token is using token_add(self, token_hash: UInt160).