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*
```
Adds syntactic comparison for some expressions, etc., allowing in
particular to detect syntactically equal expressions. Positions are,
obviously, ignored.
