Provide a new API to get Currency's account address

[moreal]

Currently, Libplanet uses new Address(Currency.Hash.ByteArray) as its currency's account address.

libplanet/src/Libplanet.Action/State/IWorldExtensions.cs

Line 211 in 2740bf1

worldState.GetAccountState(new Address(currency.Hash.ByteArray)).Trie,

The address of the Currency account is needed to understand the trie diff result, to know which account is of which currency.

If there is a reason to make opening the API difficult, I want to discuss it :)