Lua: support topdown traversals

The traversal order of filters can now be selected by setting the key
`traverse` of the filter to either `'topdown'` or `'typewise'`; the
default remains `'typewise'`.

Topdown traversals can be cut short by returning `false` as a second
value from the filter function. No child-element of the returned element
is processed in that case.

stack.yaml

@@ -26,7 +26,6 @@ extra-deps:
 - lua-2.0.2
 - tasty-hslua-1.0.0
 - tasty-lua-1.0.0
-- pandoc-lua-marshal-0.1.2
 - pandoc-types-1.22.1
 - commonmark-0.2.1.1
 - commonmark-extensions-0.2.2
@@ -37,6 +36,8 @@ extra-deps:
 - unicode-data-0.2.0
 - git: https://github.com/jgm/ipynb.git
   commit: 00246af10885c2ad4413ace4f69a7e6c88297a08
+- git: https://github.com/pandoc/pandoc-lua-marshal
+  commit: 98e3de5087bb533fadbb89720f83f7f7cc3be5bf
 ghc-options:
    "$locals": -fhide-source-paths -Wno-missing-home-modules
 resolver: lts-18.10