Turtles would have a new hidden ItemStack that stores the currently equipped tool.
Tools would use the same render code as when being rendered by a player.

boolean turtle.equip() -- equips/swaps the item at the current slot, fails if not equipable
boolean turtle.dequip() -- removes the tool to the current slot (if empty)
boolean turtle.canUse() -- also canUseUp, canUseDown true if the equipped tool is appropriate

boolean turtle.compareToName(name, [size]) -- true if the selected slot matches the name and if size ~= nil, the current stack size