Remove `DoubleEntry::currency`

The `DoubleEntry::currency` method has little reason for existence outside the gem. Externally the currency of an account can be obtained via `DoubleEntry::Account#currency` or `DoubleEntry::Account::Instance#currency`.

Let's not expose `DoubleEntry::currency` as a public API.
Account and transfer definitions are now stored in the `Account` and `Transfer` classes respectively.