A user's balance of one currency. Currencies are free-form string codes ("gold", "gems", "energy") — the game decides which exist.
"gold"
"gems"
"energy"
@type t() :: %Gamend.Economy.Wallet{ __meta__: term(), balance: term(), currency: term(), id: term(), inserted_at: term(), updated_at: term(), user: term(), user_id: term() }