Merge 'Fix bug: we cant remove order by terms from the head of the list' from Jussi Saurio

we had an incorrect optimization in `eliminate_orderby_like_groupby()`
where it could remove e.g. the first term of the ORDER BY if it matched
the first GROUP BY term and the result set was naturally ordered by that
term. this is invalid. see e.g.:
```sql
main branch - BAD: removes the `ORDER BY id` term because the results are naturally ordered by id.
However, this results in sorting the entire thing by last name only!

limbo> select id, last_name, count(1) from users GROUP BY 1,2 order by id, last_name desc limit 3;
┌──────┬───────────┬───────────┐
│ id   │ last_name │ count (1) │
├──────┼───────────┼───────────┤
│ 6235 │ Zuniga    │         1 │
├──────┼───────────┼───────────┤
│ 8043 │ Zuniga    │         1 │
├──────┼───────────┼───────────┤
│  944 │ Zimmerman │         1 │
└──────┴───────────┴───────────┘

after fix - GOOD:

limbo> select id, last_name, count(1) from users GROUP BY 1,2 order by id, last_name desc limit 3;
┌────┬───────────┬───────────┐
│ id │ last_name │ count (1) │
├────┼───────────┼───────────┤
│  1 │ Foster    │         1 │
├────┼───────────┼───────────┤
│  2 │ Salazar   │         1 │
├────┼───────────┼───────────┤
│  3 │ Perry     │         1 │
└────┴───────────┴───────────┘

I also refactored sorters to always use the ast `SortOrder` instead of boolean vectors, and use the `compare_immutable()` utility we use inside btrees too.

Closes #1365

testing/groupby.test

@@ -185,3 +185,10 @@ William|111}
 do_execsql_test group_by_column_number {
   select u.first_name, count(1) from users u group by 1 limit 1;
 } {Aaron|41}
+
+# There was a regression where we incorrectly removed SOME order by terms and left others in place, which is invalid and results in wrong rows being returned.
+do_execsql_test groupby_orderby_removal_regression_test {
+  select id, last_name, count(1) from users GROUP BY 1,2 order by id, last_name desc limit 3;
+} {1|Foster|1
+2|Salazar|1
+3|Perry|1}