[WIP] Add support for migrating liquid code away from several lax parse quirks

It transforms liquid code to equivalently parsed code, that doesn't depend
on lax parsing quirks.

Note that this transformation can hide the intent of the original code, so
it is best to review the changes, since removed syntax often would indicate
a bug in the original liquid code.

lib/liquid/range_lookup.rb

@@ -22,6 +22,37 @@ module Liquid
       end
     end
 
+    def self.lax_migrate(start_markup, end_markup)
+      new_start = Expression.lax_migrate(start_markup)
+      new_end = Expression.lax_migrate(end_markup)
+
+      # cast literals
+      start_obj = Expression.parse(new_start)
+      end_obj   = Expression.parse(new_end)
+      if start_obj.respond_to?(:evaluate) || end_obj.respond_to?(:evaluate)
+        new_start = lax_migrate_range_expression(new_start, start_obj)
+        new_end = lax_migrate_range_expression(new_end, end_obj)
+      else
+        new_start = start_obj.to_i.to_s unless start_obj.is_a?(Integer)
+        new_end = end_obj.to_i.to_s unless end_obj.is_a?(Integer)
+      end
+
+      [new_start, new_end]
+    end
+
+    def self.lax_migrate_range_expression(markup, expression)
+      return markup if expression.respond_to?(:evaluate)
+
+      case expression
+      when Integer
+        markup
+      when NilClass, String
+        expression.to_i.to_s
+      else
+        Utils.to_integer(input).to_s
+      end
+    end
+
     attr_reader :start_obj, :end_obj
 
     def initialize(start_obj, end_obj)