Lua: add module for AST element sequence traversal

Lua filters must be able to traverse sequences of AST elements and to
replace elements by splicing sequences back in their place. Special
`Walkable` instances can be used for this; those are provided in a new
module `Text.Pandoc.Lua.Walk`.

stack.yaml

@@ -14,6 +14,8 @@ extra-deps:
 # - pandoc-citeproc-0.16.2
 - git: https://github.com/jgm/pandoc-citeproc
   commit: 6d62678ece91bbb4fe4f5a99695006e1d53c3bae
+- git: https://github.com/tarleb/pandoc-types
+  commit: a087b0174a597b92c5fec4d633c46887c188b496
 - ipynb-0.1
 - cmark-gfm-0.2.0
 - hslua-1.0.3.1