Resolve aliases in cost calculation

* If stairs from stairs mod were crafted before moreblocks was enabled,
  the old (now aliased) stairs variant is kept in inventory. If the player
  tried to recycle that stair variant, `inv:contains_item("output", stackname)`
  would return true as `contains_item` resolves alias names,
  but `circular_saw:get_cost` would return nil as it's expecting exact
  `stackname` matches which would later crash due to nil arithmetic.
  This PR fixes this issue by resolving alias using `minetest.registered_aliases` table.
  `circular_saw:get_cost` should work now in every scenario where
  `inv:contains_item("output", stackname)` returns true.
* backported from minetest-mods#175
* fixes minetest-mods#174

circular_saw.lua

@@ -89,8 +89,9 @@ circular_saw.names = {
 }
 
 function circular_saw:get_cost(inv, stackname)
+	local name = minetest.registered_aliases[stackname] or stackname
 	for i, item in pairs(inv:get_list("output")) do
-		if item:get_name() == stackname then
+		if item:get_name() == name then
 			return circular_saw.cost_in_microblocks[i]
 		end
 	end