SteamAchievement.is_unlocked is unnecessarily costly

[smiley]

"is_unlocked" fetches the entire achievement unlocks list for its game, then only uses its own value. While this value is then cached (so the same achievement's status doesn't change as long as the achievements list isn't re-fetched), asking another achievement the same thing will cause unnecessary slowdowns.

There needs to be a way to fetch achievements for an entire game, not through the game object itself. Maybe two-way binding is the right solution, but that might screw up garbage collection.