The huge benefit of this approach is that almost no changes are needed and we get compatible types between dcalc and lcalc, allowing to deduplicate a few functions.
It might not be the best in the long run: there are still benefits in factorising small parts of the AST as suggested in #157, and this forces a central AST definition that makes the nanopass-like approach a bit less legible.
Still, I think it's a step in the right direction and it doesn't really lock us in keeping to use the big GADT (as the minimal cascade of changes show).
It's not expected to stay that way forever, but some additional effort will be required for them to preserve (or restore) types; until then, be safe and don't forward possibly incorrect information.
The AST structures track annotations (e.g., at the moment, source code
position information) in a lot of places. This patch tidies up a bit and
removes some duplication, ensuring a single level of annotation wrapping
at each AST recursion level.
This will be important when adding type information in these
annotations, because there will be consitency constraints to be ensured
and duplication is a likely source of mistakes.
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*
```
