Implement total() aggregation function

- Returns 0.0 when called on non integer / non float columns
- Always returns floating point number
- fix: default for sum() should be NULL when there is no non-NULL row
  per docs

testing/all.test

@@ -41,6 +41,10 @@ do_execsql_test select-sum {
   SELECT sum(age) FROM users;
 } {503960}
 
+do_execsql_test select-total {
+  SELECT sum(age) FROM users;
+} {503960}
+
 do_execsql_test select-limit {
   SELECT id FROM users LIMIT 1;
 } {1}