Update drowning

Needed if #851 gets merged
Fix typo in flower generation
Add mapgen_stair_cobble to minetest_game
Use 6d facedir to place upside down slabs and stairs
Longer range and insta-dig in creative
Remove mapgen_air alias
Change hoe_on_use to farming.hoe_on_use, for use and override by mods.
Add and use the stick group
Remove trailing tabs from empty lines in stairs
Fix stone and wooden axe being slower than hand
Move sapling growing and grass adding/removing ABMs to Lua.
Fix grass adding/removing ABM.
Fix possible crash with grass ABM.
Implemented facedir in the nyan cat rainbow
Revert dig time in creative mode to 0.5

This made it impossible to punch things without breaking them in creative
Fix tree growing and jungle sapling growth rate