Cache Journey Node#to_s

Even though the generation has been optimized, it is wasteful
to keep generating the same string again and again.

Initially I wasn't really for adding this cache, but Matthew
pointed that the `uri_pattern` is accessed by Open Telemetry,
so lazily memoizing seem sound.

Author: byroot

actionpack/lib/action_dispatch/journey/nodes/node.rb

@@ -74,14 +74,15 @@ class Node # :nodoc:
         def initialize(left)
           @left = left
           @memo = nil
+          @to_s = nil
         end
 
         def each(&block)
           Visitors::Each::INSTANCE.accept(self, block)
         end
 
         def to_s
-          Visitors::String::INSTANCE.accept(self, "".dup)
+          @to_s ||= Visitors::String::INSTANCE.accept(self, "".dup).freeze
         end
 
         def to_dot