Optimize #parent_id Ancestry instance_method

If @ancestor_ids has already been calculated, using that value will be
much more efficient for calculating the parent since the work translate
the string to the proper typecasted value has already been done.

But if it hasn't (for example, when calling `.arrange_nodes`), then
ancestor_ids is never actually needed in full, and we can avoid the
object allocations and CPU cycles needed to determine the depth by
typecasting all of the ids in the string and just use a bit of substring
manipulation to only allocate one object prior to typecasting (instead
of N+1 for number of ancestors).

Also, since it is possible for `@_parent_id` to be `nil`, make sure we
are removing the instance variable instead of just setting it to `nil`
if the location in the tree has been changed.

lib/patches/ancestry_patch.rb

@@ -24,6 +24,23 @@ def ancestor_ids
       @_ancestor_ids ||= parse_ancestry_column(read_attribute(ancestry_base_class.ancestry_column))
     end
 
+    def parent_id
+      return @_parent_id if defined?(@_parent_id)
+      @_parent_id = if @ancestor_ids
+                      @ancestor_ids.empty? ? nil : @ancestor_ids.last
+                    else
+                      col = read_attribute(ancestry_base_class.ancestry_column)
+                      # Specifically not using `.blank?` here because it is
+                      # slower than doing the below.
+                      if col.nil? || col.empty? # rubocop:disable Rails/Blank
+                        nil
+                      else
+                        rindex = col.rindex(ANCESTRY_DELIMITER)
+                        cast_primary_key(rindex ? col[rindex + 1, col.length] : col)
+                      end
+                    end
+    end
+
     def depth
       @_depth ||= if @ancestor_ids
                     @ancestor_ids.size
@@ -45,6 +62,9 @@ def cast_primary_key(key)
     def clear_memoized_instance_variables
       @_ancestor_ids = nil
       @_depth        = nil
+
+      # can't assign to `nil` since `nil` could be a valid result
+      remove_instance_variable(:@_parent_id) if defined?(@_parent_id)
     end
   end
 end