Merge 'Add support for DISTINCT aggregate functions' from Jussi Saurio

Reviewable commit by commit. CI failures are not related.
Adds support for e.g. `select first_name, sum(distinct age),
count(distinct age), avg(distinct age) from users group by 1`
Implementation details:
- Creates an ephemeral index per distinct aggregate, and jumps over the
accumulation step if a duplicate is found

Closes #1507

testing/agg-functions.test

@@ -127,3 +127,7 @@ do_execsql_test select-agg-json-array {
 do_execsql_test select-agg-json-array-object {
   SELECT json_group_array(json_object('name', name)) FROM products;
 } {[{"name":"hat"},{"name":"cap"},{"name":"shirt"},{"name":"sweater"},{"name":"sweatshirt"},{"name":"shorts"},{"name":"jeans"},{"name":"sneakers"},{"name":"boots"},{"name":"coat"},{"name":"accessories"}]}
+
+do_execsql_test select-distinct-agg-functions {
+  SELECT sum(distinct age), count(distinct age), avg(distinct age) FROM users;
+} {5050|100|50.5}

testing/groupby.test

@@ -197,4 +197,13 @@ do_execsql_test group_by_no_sorting_required {
   select age, count(1) from users group by age limit 3;
 } {1|112
 2|113
-3|97}
+3|97}
+
+do_execsql_test distinct_agg_functions {
+  select first_name, sum(distinct age), count(distinct age), avg(distinct age) 
+  from users 
+  group by 1 
+  limit 3;
+} {Aaron|1769|33|53.6060606060606
+Abigail|833|15|55.5333333333333
+Adam|1517|30|50.5666666666667}