Make route mapping scope lookups faster

Scopes are backed by an array of hashes, but we could make lookup faster
by just merging inherited values into the immediate hash. This us from
needless iterations for deeply nested routes.

Author: gmcgibbon

actionpack/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Speed up `ActionDispatch::Routing::Mapper::Scope#[]` by merging frame hashes.
+
+    *Gannon McGibbon*
+
 *   Allow bots to ignore `allow_browser`.
 
     *Matthew Nguyen*

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -2293,9 +2293,9 @@ class Scope # :nodoc:
 
         attr_reader :parent, :scope_level
 
-        def initialize(hash, parent = NULL, scope_level = nil)
-          @hash = hash
+        def initialize(hash, parent = ROOT, scope_level = nil)
           @parent = parent
+          @hash = parent ? parent.frame.merge(hash) : hash
           @scope_level = scope_level
         end
 
@@ -2308,7 +2308,7 @@ def null?
         end
 
         def root?
-          @parent.null?
+          @parent == ROOT
         end
 
         def resources?
@@ -2353,23 +2353,22 @@ def new_level(level)
         end
 
         def [](key)
-          scope = find { |node| node.frame.key? key }
-          scope && scope.frame[key]
+          frame[key]
         end
 
+        def frame; @hash; end
+
         include Enumerable
 
         def each
           node = self
-          until node.equal? NULL
+          until node.equal? ROOT
             yield node
             node = node.parent
           end
         end
 
-        def frame; @hash; end
-
-        NULL = Scope.new(nil, nil)
+        ROOT = Scope.new({}, nil)
       end
 
       def initialize(set) # :nodoc: