Lua: update to latest pandoc-lua-marshal (0.1.1)

- `walk` methods are added to `Block` and `Inline` values; the methods
  are similar to `pandoc.utils.walk_block` and
  `pandoc.utils.walk_inline`, but apply to filter also to the element
  itself, and therefore return a list of element instead of a single
  element.

- Functions of name `Doc` are no longer accepted as alternatives for
  `Pandoc` filter functions. This functionality was undocumented.

test/lua/implicit-doc-filter.lua

@@ -1,4 +1,4 @@
-function Doc (doc)
+function Pandoc (doc)
   local meta = {}
   local hello = { pandoc.Str "Hello,", pandoc.Space(), pandoc.Str "World!" }
   local blocks = { pandoc.Para(hello) }