Memoized reflections accessor

Its value never change since associations are defined at class load time
so there is no need to build the hash everytime the method is called.

Before this change:

    Calculating -------------------------------------
             reflections   804.000  i/100ms
    -------------------------------------------------
             reflections      8.213k (±26.2%) i/s -     36.180k

After this change:

    Calculating -------------------------------------
             reflections    24.548k i/100ms
    -------------------------------------------------
             reflections      1.591M (±25.7%) i/s -      7.364M

Benchmark script:

    require 'bundler/setup'

    require 'active_record'
    require 'benchmark/ips'

    ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
    ActiveRecord::Migration.verbose = false

    ActiveRecord::Schema.define do
      100.times do |i|
        create_table "users#{i}", force: true
      end

      create_table :cars, force: true do |t|
        100.times do |i|
          t.references "users#{i}"
        end
      end
    end

    class Car < ActiveRecord::Base
      100.times do |i|
        belongs_to "users#{i}".to_sym
      end
    end

    Benchmark.ips do |x|
      x.report('reflections') { Car.reflections }
    end

activerecord/lib/active_record/reflection.rb

@@ -32,6 +32,7 @@ module ActiveRecord
     end
 
     def self.add_reflection(ar, name, reflection)
+      ar.clear_reflections_cache
       ar._reflections = ar._reflections.merge(name.to_s => reflection)
     end
 
@@ -67,18 +68,22 @@ module ActiveRecord
       #
       # @api public
       def reflections
-        ref = {}
-        _reflections.each do |name, reflection|
-          parent_reflection = reflection.parent_reflection
+        @__reflections ||= begin
+          ref = {}
 
-          if parent_reflection
-            parent_name = parent_reflection.name
-            ref[parent_name.to_s] = parent_reflection
-          else
-            ref[name] = reflection
+          _reflections.each do |name, reflection|
+            parent_reflection = reflection.parent_reflection
+
+            if parent_reflection
+              parent_name = parent_reflection.name
+              ref[parent_name.to_s] = parent_reflection
+            else
+              ref[name] = reflection
+            end
           end
+
+          ref
         end
-        ref
       end
 
       # Returns an array of AssociationReflection objects for all the
@@ -118,6 +123,10 @@ module ActiveRecord
       def reflect_on_all_autosave_associations
         reflections.values.select { |reflection| reflection.options[:autosave] }
       end
+
+      def clear_reflections_cache #:nodoc:
+        @__reflection = nil
+      end
     end
 
     # Holds all the methods that are shared between MacroReflection, AssociationReflection