Support use of LinkList positions in queries

[astigsen]

Support querying into specific positions (First, Last, 2nd...) in LinkLists. This is needed when querying into LinkLists where the user attach some meaning to the ordering (could be priority queues, highscore lists...).

In NSPredicates it is expressed as "friends[1] == %@", MySecondFriend or "friends[LAST].age > 16".

Suggested syntax:

size_t match1 = (table->column<LinkList>(linklist_col_id)[1] == MySecondFriend).find();
size_t match2 = (table->column<LinkList>(linklist_col_id).last().column<int>(age_col_id) > 16).find();

See also iOS issues:
realm/realm-swift#1490

@rrrlasse

[astigsen]

Duplicate of #4269