Use lua bools and strings for MetaBool, MetaString

Native lua booleans and strings are used to represent MetaBool and
MetaString values.  This is more natural than the previous table-based
representation.  The old lua representation can still be read back to
haskell, ensuring compatibility with the `pandoc.MetaBool` and
`pandoc.MetaString` lua constructors.

src/Text/Pandoc/Lua/StackInstances.hs

@@ -65,11 +65,11 @@ instance StackValue Meta where
 instance StackValue MetaValue where
   push lua = \case
     MetaBlocks blcks  -> pushViaConstructor lua "MetaBlocks" blcks
-    MetaBool b        -> pushViaConstructor lua "MetaBool" b
+    MetaBool bool     -> push lua bool
     MetaInlines inlns -> pushViaConstructor lua "MetaInlines" inlns
     MetaList metalist -> pushViaConstructor lua "MetaList" metalist
     MetaMap metamap   -> pushViaConstructor lua "MetaMap" metamap
-    MetaString cs     -> pushViaConstructor lua "MetaString" cs
+    MetaString str    -> push lua str
   peek lua idx = do
     luatype <- ltype lua idx
     case luatype of