Revert "Merge pull request #466 from Shopify/remove-expression-cache"

This reverts commit d9ae36ec40, reversing
changes made to 2da9d49478.

lib/liquid/context.rb

@@ -24,6 +24,7 @@ module Liquid
       @resource_limits  = resource_limits || Template.default_resource_limits.dup
       @resource_limits[:render_score_current] = 0
       @resource_limits[:assign_score_current] = 0
+      @parsed_expression = Hash.new{ |cache, markup| cache[markup] = Expression.parse(markup) }
       squash_instance_assigns_with_environments
 
       @this_stack_used = false
@@ -169,7 +170,7 @@ module Liquid
     # Example:
     #   products == empty #=> products.empty?
     def [](expression)
-      evaluate(Expression.parse(expression))
+      evaluate(@parsed_expression[expression])
     end
 
     def has_key?(key)