Allow fragments to be restored to crystal form.

As disused at https://github.com/minetest/minetest_game/pull/133 , current mese fragment behaver is inconsistent and a bit odd, considering mese fragments serve no function whatsoever.

This adds a recipe to change mese fragments back into mese crystals, so they can be used again.

This would give mese fragments value on economy-based servers. As it is, mese fragments are not even good for trading, because no one wants to trade for them. After all, once they have enough of them, they still can't be formed back into crystals, or any other usable form.