Lua: traverse nested blocks and inlines in correct order

Traversal methods are updated to use the new Walk module such that
sequences with nested Inline (or Block) elements are traversed in the
order in which they appear in the linearized document.

Fixes: #5667

test/lua/module/pandoc.lua

@@ -171,4 +171,42 @@ return {
       )
     end)
   },
+
+  group 'walk_block' {
+    test('block walking order', function ()
+     local acc = {}
+     local nested_nums = pandoc.Div {
+       pandoc.Para{pandoc.Str'1'},
+       pandoc.Div{
+         pandoc.Para{pandoc.Str'2'},
+         pandoc.Para{pandoc.Str'3'}
+       },
+       pandoc.Para{pandoc.Str'4'}
+     }
+     pandoc.walk_block(
+       nested_nums,
+       {Para = function (p) table.insert(acc, p.content[1].text) end}
+     )
+     assert.are_equal('1234', table.concat(acc))
+    end)
+  },
+
+  group 'walk_inline' {
+    test('inline walking order', function ()
+      local acc = {}
+      local nested_nums = pandoc.Span {
+        pandoc.Str'1',
+        pandoc.Emph {
+          pandoc.Str'2',
+          pandoc.Str'3'
+        },
+        pandoc.Str'4'
+      }
+      pandoc.walk_inline(
+        nested_nums,
+        {Str = function (s) table.insert(acc, s.text) end}
+      )
+      assert.are_equal('1234', table.concat(acc))
+    end)
+  }
 }