implement matchmaking lobby

Fixes #2.

game/player.go

@@ -41,6 +41,12 @@ func (p *Player) DTO() serve.Player {
 	return r
 }
 
+// Is returns whether the player has the same ID as other.
+// This exists for tests.
+func (p *Player) Is(other *Player) bool {
+	return p.id == other.id
+}
+
 type CuffState uint8
 
 const (