data/pandoc.lua: auto-fix nested constructor arguments

Incorrect types to pandoc element constructors are automatically
converted to the correct types when possible. This was already done for
most constructors, but conversions are now also done for nested
types (like lists of lists).

test/lua/smart-constructors.lua

@@ -0,0 +1,10 @@
+-- Test that constructors are "smart" in that they autoconvert
+-- types where sensible.
+function Para (_)
+  return {
+    pandoc.BulletList{pandoc.Para "Hello", pandoc.Para "World"},
+    pandoc.DefinitionList{{"foo", pandoc.Para "placeholder"}},
+    pandoc.LineBlock{"Moin", "Welt"},
+    pandoc.OrderedList{pandoc.Plain{pandoc.Str "one"}, pandoc.Plain "two"}
+  }
+end