Support named tuples as return values

[cmd410]

So, to be frank, this PR adds the ability to define tuples for DB models beforehand and use them as return types: consider the following example:

import easy_sqlite3

type
  User = tuple[id: int, username: string, email: string]

proc createUsersTable*() {.importdb: staticRead("sql/create_users_table.sql").}
proc insertUser*(username, email: string) {.importdb: staticRead("sql/insert_user.sql").}
proc selectUserById*(id: int): User {.importdb: staticRead("sql/select_user_by_id.sql").}

selectUserById has return type annotated as User. It wasn't possible before because importdb expected only int as identifier there, so the user had no choice but to copy and paste the whole tuple definition.

Now by contrast it can be any identifier which is either int or evaluates to tuple[...] type.

It is achieved by making body parameter in importdb be typed, so that compiler binds type annotations for the proc, which we can get implementation of later.

I ran the tests with creating tables, inserting, selecting and updating data. All work as intended, should be backwards compatible, I guess.

[codehz]

Cool! thanks