Show item usages in craft guide

When the craft guide is showing a craft, the output slot is now a button,
which causes the craft guide to show ways in which that output can be
used.  This mirrors the way input slots are buttons that show recipes
for the selected ingredient.  Usages of an item can be iterated through
in the same way as recipes for the item.  This incidentally offers some
ability to retrace one's steps through a crafting chain, without storing
actual history.

group.lua

@@ -1,3 +1,25 @@
+function unified_inventory.canonical_item_spec_matcher(spec)
+	local specname = ItemStack(spec):get_name()
+	if specname:sub(1, 6) == "group:" then
+		local group_names = specname:sub(7):split(",")
+		return function (itemname)
+			local itemdef = minetest.registered_items[itemname]
+			for _, group_name in ipairs(group_names) do
+				if (itemdef.groups[group_name] or 0) == 0 then
+					return false
+				end
+			end
+			return true
+		end
+	else
+		return function (itemname) return itemname == specname end
+	end
+end
+
+function unified_inventory.item_matches_spec(item, spec)
+	local itemname = ItemStack(item):get_name()
+	return unified_inventory.canonical_item_spec_matcher(spec)(itemname)
+end
 
 unified_inventory.registered_group_items = {
 	mesecon_conductor_craftable = "mesecons:wire_00000000_off",