Tar blocks

Add some tar blocks, to trap and slow players and mobs. They can be used to create bottlenecks and traps. Tar could be found in lakes in desert biomes, and could require buckets to move, like water and lava. It would behave more like regular blocks, not flowing like lava and water. It could be flammable, but burns indefinitely, like netherack.

Could be combined with gravel to make tarmac.