lib: add a field to 'Journal' for a stack of include files

- In anticipation of megaparsec 7, which removes support for stacks of
  include files (as far as I can tell)
- Intended for the 'StateT Journal' layer of the parser
  - A stack of include files would be better in a 'ReaderT' layer, but I
    don't want to add another layer to the parser
- Intended for detecting cycles of include files
  - Potential issue: for proper error messages for include file cycles,
    we must remember to provide the filepath of the root journal file via
    the initial journal state passed to a 'JournalParser'; I imagine
    that we may forget to do so because in all other cases it is okay
    not to do so.

hledger-lib/Hledger/Data/Journal.hs

@@ -163,6 +163,7 @@ instance Sem.Semigroup Journal where
     ,jparsealiases              = jparsealiases              j2
     -- ,jparsetransactioncount     = jparsetransactioncount     j1 +  jparsetransactioncount     j2
     ,jparsetimeclockentries = jparsetimeclockentries j1 <> jparsetimeclockentries j2
+    ,jincludefilestack          = jincludefilestack          j2
     ,jdeclaredaccounts                  = jdeclaredaccounts                  j1 <> jdeclaredaccounts                  j2
     ,jcommodities               = jcommodities               j1 <> jcommodities               j2
     ,jinferredcommodities       = jinferredcommodities       j1 <> jinferredcommodities       j2
@@ -189,8 +190,9 @@ nulljournal = Journal {
   ,jparseparentaccounts       = []
   ,jparsealiases              = []
   -- ,jparsetransactioncount     = 0
-  ,jparsetimeclockentries = []
-  ,jdeclaredaccounts                  = []
+  ,jparsetimeclockentries     = []
+  ,jincludefilestack          = []
+  ,jdeclaredaccounts          = []
   ,jcommodities               = M.fromList []
   ,jinferredcommodities       = M.fromList []
   ,jmarketprices              = []

hledger-lib/Hledger/Data/Types.hs

@@ -363,6 +363,7 @@ data Journal = Journal {
   ,jparsealiases          :: [AccountAlias]                        -- ^ the current account name aliases in effect, specified by alias directives (& options ?)
   -- ,jparsetransactioncount :: Integer                               -- ^ the current count of transactions parsed so far (only journal format txns, currently)
   ,jparsetimeclockentries :: [TimeclockEntry]                       -- ^ timeclock sessions which have not been clocked out
+  ,jincludefilestack      :: [FilePath]
   -- principal data
   ,jdeclaredaccounts      :: [AccountName]                          -- ^ Accounts declared by account directives, in parse order (after journal finalisation) 
   ,jcommodities           :: M.Map CommoditySymbol Commodity        -- ^ commodities and formats declared by commodity directives

hledger-lib/Hledger/Read/Common.hs

@@ -226,7 +226,10 @@ parseAndFinaliseJournal :: JournalParser IO ParsedJournal -> InputOpts
 parseAndFinaliseJournal parser iopts f txt = do
   t <- liftIO getClockTime
   y <- liftIO getCurrentYear
-  ep <- liftIO $ runParserT (evalStateT parser nulljournal {jparsedefaultyear=Just y}) f txt
+  let initJournal = nulljournal
+        { jparsedefaultyear = Just y
+        , jincludefilestack = [f] }
+  ep <- liftIO $ runParserT (evalStateT parser initJournal) f txt
   case ep of
     Right pj -> 
       let pj' = if auto_ iopts then applyTransactionModifiers pj else pj in