Slabbed

Fixed: the game no longer crashes on startup with Lithium installed! Both mods were trying to rewrite the same piece of Minecraft's explosion code, so Slabbed now steps aside there while Lithium is present - explosions behind a lowered block follow normal Minecraft rules with Lithium installed, and everything else (arrows, mob sight, collision) keeps working the way it should.

Also fixed: with Lithium installed you could walk through the lower half of a lowered block, because Lithium's faster collision search never saw the part that hangs below the grid cell. Slabbed now adds that part to Lithium's search too. One tiny edge remains: a lowered block hanging into a completely empty chunk section can still be missed by Lithium's search.

Thanks to everyone who reported the crash on GitHub (#74)!