journal: add new state field for unassignable accounts

This should eventually include accountnames from transaction
modifiers (if `--auto` is enabled), or periodic transactions (if
`--forecast` is enabled).

hledger-lib/Hledger/Data/Journal.hs

@@ -97,6 +97,7 @@ import Data.Tree
 import System.Time (ClockTime(TOD))
 import Text.Printf
 import qualified Data.Map as M
+import qualified Data.Set as S
 
 import Hledger.Utils 
 import Hledger.Data.Types
@@ -641,6 +642,7 @@ journalBalanceTransactionsST assrt j createStore storeIn extract =
                   (storeIn txStore) 
                   assrt
                   (Just $ journalCommodityStyles j)
+                  S.empty
     flip R.runReaderT env $ do
       dated <- fmap snd . sortBy (comparing fst) . concat
                 <$> mapM' discriminateByDate (jtxns j)
@@ -659,6 +661,7 @@ data Env s = Env { eBalances :: HT.HashTable s AccountName MixedAmount
                  , eStoreTx      :: Transaction -> ST s ()
                  , eAssrt        :: Bool
                  , eStyles       :: Maybe (M.Map CommoditySymbol AmountStyle)
+                 , eUnassignable :: S.Set AccountName
                  }
 
 -- | This converts a transaction into a list of transactions or