Move line_item_comparison_hooks config to Spree::Config

Setting line item comparison hooks in a `config.to_prepare` block will
currently autoload the Spree::Order model (and subsequently any model
referenced in that class definition), slowing down the boot process in
development mode (when we want our app/ directory to be autoloaded.

This moves setting those hooks to Spree::Config, which is already loaded
by the time initializers run. This allows us to move code out of
`config.to_prepare` hooks, and stop the order class from being
inadvertently eager loaded.

core/app/models/spree/order.rb

@@ -163,8 +163,16 @@ def self.find_by_param!(value)
     delegate :name, to: :bill_address, prefix: true, allow_nil: true
     alias_method :billing_name, :bill_address_name
 
-    class_attribute :line_item_comparison_hooks
-    self.line_item_comparison_hooks = Set.new
+    def line_item_comparison_hooks
+      Set.new(Spree::Config.line_item_comparison_hooks)
+    end
+
+    class << self
+      def line_item_comparison_hooks=(value)
+        Spree::Config.line_item_comparison_hooks = value.to_a
+      end
+      deprecate :line_item_comparison_hooks=, deprecator: Spree.deprecator
+    end
 
     scope :created_between, ->(start_date, end_date) { where(created_at: start_date..end_date) }
     scope :completed_between, ->(start_date, end_date) { where(completed_at: start_date..end_date) }

core/lib/spree/app_configuration.rb

@@ -168,6 +168,13 @@ class AppConfiguration < Preferences::Configuration
     #   @return [String] template to use for layout on the frontend (default: +"spree/layouts/spree_application"+)
     preference :layout, :string, default: 'spree/layouts/spree_application'
 
+    # !@attribute [rw] line_item_comparison_hooks
+    #   @return [Array<Symbol>] An array of methods to call on {Spree::Order} to determine if a line item is equal to another
+    #   (default: +[]+)
+    #   @example
+    #   config.line_item_comparison_hooks << :my_custom_method
+    preference :line_item_comparison_hooks, :array, default: []
+
     # @!attribute [rw] logo
     #   @return [String] URL of logo used on frontend (default: +'logo/solidus.svg'+)
     preference :logo, :string, default: 'logo/solidus.svg'

core/spec/models/spree/order_merger_spec.rb

@@ -67,12 +67,7 @@ module Spree
 
     context "merging using extension-specific line_item_comparison_hooks" do
       before do
-        Spree::Order.register_line_item_comparison_hook(:foos_match)
-      end
-
-      after do
-        # reset to avoid test pollution
-        Spree::Order.line_item_comparison_hooks = Set.new
+        stub_spree_preferences(line_item_comparison_hooks: [:foos_match])
       end
 
       context "2 equal line items" do

core/spec/models/spree/order_spec.rb

@@ -741,12 +741,7 @@ def merge!(other_order, user = nil)
 
     context "match line item with options", partial_double_verification: false do
       before do
-        Spree::Order.register_line_item_comparison_hook(:foos_match)
-      end
-
-      after do
-        # reset to avoid test pollution
-        Spree::Order.line_item_comparison_hooks = Set.new
+        stub_spree_preferences(line_item_comparison_hooks: [:foos_match])
       end
 
       it "matches line item when options match" do

promotions/lib/solidus_promotions/engine.rb

@@ -43,10 +43,7 @@ class Engine < Rails::Engine
 
     initializer "solidus_promotions.spree_config", after: "spree.load_config_initializers" do
       Spree::Config.adjustment_promotion_source_types << "SolidusPromotions::Benefit"
-
-      Rails.application.config.to_prepare do
-        Spree::Order.line_item_comparison_hooks << :free_from_order_benefit?
-      end
+      Spree::Config.line_item_comparison_hooks << :free_from_order_benefit?
     end
 
     initializer "solidus_promotions.core.pub_sub", after: "spree.core.pub_sub" do |app|