Support for CURSOR

Nobody has claimed this yet.

A `CURSOR` would allow us to paginate really large collections.

I've never implemented or even really used these directly, but I found this example

```sql
-- We must be in a transaction
BEGIN;
-- Open a cursor for a query
DECLARE medley_cur CURSOR FOR SELECT * FROM medley;
-- Retrieve ten rows
FETCH 10 FROM medley_cur;
-- ...
-- Retrieve ten more from where we left off
FETCH 10 FROM medley_cur;
-- All done
COMMIT;
```

I honestly have no clue how this will work, but I think we would probably want some alternate method of iterating that appends the cursor to the query?

```crystal
UserQuery.new.each_with_cursor(fetch: 10) do |user|
#??
end

UserQuery.new.cursor_fetch(10).each do |user|

end
```

If anyone has any ideas around this, I'm open.

The issue names no implementation files, tests, or entry points. Start by reviewing Avram's existing query iteration API and the PostgreSQL cursor requirements, then establish the API and transaction behavior for cursor pagination. Done means large collections can be paginated through a tested cursor workflow.

Written by the indexing model from the issue text.