This class had slightly confusing semantics and the added weirdness
doesn't seem worth it just so we can say "." instead of "->" when
iterating over a vector of NNRPs.
This patch replaces NonnullRefPtrVector<T> with Vector<NNRP<T>>.
Because `card->value() == 11` is a lot less clear than `card->rank() ==
Cards::Rank::Queen`, and also safer.
Put this, along with the `Suit` enum, in the `Cards` namespace directly
instead of inside `Cards::Card`. Slightly less typing that way.
Playing cards have a `suit` such as `hearts`/`diamonds`, not a
`type`. Make the internal naming consistent with the way playing cards
are typically named.
When we don't have a matching card for the lead card rather than
always preferring to play hearts we should try to get rid of our
high value cards first if no other player has hearts cards higher
than what we have.
Previously the AI would prefer playing a lead card for which no other
player had a card with a higher value even though it also had a card
for which a higher value card was still in play.
Instead of picking the card with the lowest value we should pick the
card with the highest value for which we know no lower value card is
in play anymore and that someone else still has an even higher value
card.
