Refactor/export md using tree type

[RCmerci]

• markdown.ml & tree_type.ml
  • [X] move out module Output
  • [X] replace embed&block-ref in tree-type
• zip.ml
  • [X] add insert_lefts&insert_rights
  • [X] add type signature
• opml.ml
  • [X] replace embed & block-ref in opml