Add ParseContext#new_block_body to centralize the liquid-c override point

lib/liquid/block.rb

@@ -59,7 +59,7 @@ module Liquid
 
     # @api public
     def new_body
-      BlockBody.new
+      parse_context.new_block_body
     end
 
     # @api public

lib/liquid/document.rb

@@ -48,7 +48,7 @@ module Liquid
     private
 
     def new_body
-      Liquid::BlockBody.new
+      parse_context.new_block_body
     end
   end
 end

lib/liquid/parse_context.rb

@@ -19,6 +19,10 @@ module Liquid
       @options[option_key]
     end
 
+    def new_block_body
+      Liquid::BlockBody.new
+    end
+
     def partial=(value)
       @partial = value
       @options = value ? partial_options : @template_options