the small downside is that we lose a little bit of granularity by making
defining all the types at once (with `type ... and`) ; but one well-placed
annotation seems to be enough.
also discarded the `iter` visitor that was unused.
The module is renamed to `Mark`, and functions renamed to avoid redundancy:
`Marked.mark` is now `Mark.add`
`Marked.unmark` is now `Mark.remove`
`Marked.map_under_mark` is now simply `Mark.map`
etc.
`Marked.same_mark_as` is replaced by `Mark.copy`, but with the arguments
swapped (which seemed more convenient throughout)
Since a type `Mark.t` would indicate a mark, and to avoid confusion, the type
`Marked.t` is renamed to `Mark.ed` as a shorthand for `Mark.marked` ; this part
can easily be removed if that's too much quirkiness.
this patch is just a bunch of `sed` commands
```shell
cd compiler
sed -i 's/Pos.marked/Marked.pos/g' *.ml* **/*.ml*
sed -i 's/Pos.unmark/Marked.unmark/g' *.ml* **/*.ml*
sed -i 's/Pos\.get_position/Marked.get_mark/g' *.ml* **/*.ml*
sed -i 's/Pos\.same_pos_as/Marked.same_mark_as/g' *.ml* **/*.ml*
sed -i 's/Pos\.map_under_mark/Marked.map_under_mark/g' *.ml* **/*.ml*
sed -i 's/Pos\.mark/Marked.mark/g' *.ml* **/*.ml*
sed -i 's/Pos\.compare_marked/Marked.compare/g' *.ml* **/*.ml*
```
