Reversing unordered requests

[groue]

This PR has the request.reversed() method do nothing unless an ordering was already applied:

// Unchanged: SELECT * FROM players ORDER BY score DESC
Player.order(Column("score")).reversed()

// New: SELECT * FROM players
Player.all().reversed()

In previous versions, reversed() would sort by descending rowId (inspiration drawn from ActiveRecord). Unfortunately, this made-up ordering is not guaranteed to be the exact opposite of the default SQLite ordering. And associations will soon need a stricter model of what "reversing" a request means.

So let's go back to basics:

If a SELECT statement that returns more than one row does not have an ORDER BY clause, the order in which the rows are returned is undefined.

Reversing an undefined order gives another undefined order.