implement DEFAULT, and default syntax {expr}??

[jongleb]

Description

This PR adds support to DEFAULT and special {expr}?? syntax that generates a regular option when insert.

The main motivation is to not make DEFAULT part of regular expr, because in sql this DEFAULT is only available on insert and update.

That is, writing like this will not be valid and an error will be caught:

SELECT ... WHERE {@x + 2}??

On the contrary, using this on insert/update will just generate option, and if there is no data, DEFAULT will be generated, similar to how it works for filtering and skipping {expr}? syntax.

example taken from tests:

INSERT INTO `registration_feedbacks`
SET
  `user_message` = { CONCAT(@user_message, '22222') }??,
  `grant_types` = { @grant_types { A {'2'} | B {'2'} } }??;

The generated result:

T.execute db ("INSERT INTO `registration_feedbacks`\n\
SET\n\
  `user_message` = " ^ (match user_message with Some _ -> " ( " ^ " CONCAT(" ^ "?" ^ ", '22222') " ^ " ) " | None -> " DEFAULT ") ^ ",\n\
  `grant_types` = " ^ (match grant_types with Some (grant_types) -> " ( " ^ " " ^ (match grant_types with `A -> "'2'" | `B -> "'2'") ^ " " ^ " ) " | None -> " DEFAULT ")) set_params