- `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.