move all HashWithIndifferentAccess recursive enhancement over to the Util class

TODO: moving this two a class method on HashWithIndifferentAccess would really make the most sense, but there are two blockers:
- historically the method has operated out of Util. While totally ok, simply dedeferring to HashWithIndifferentAccess feels a bit awkward
- the contracts gem doesn’t support class method at this time. the assurance of frozen output is important considering the uses of the recursively_enhance method

middleman-core/lib/middleman-core/util/hash_with_indifferent_access.rb

@@ -19,7 +19,7 @@ module Middleman
         super()
 
         hash.each do |key, val|
-          self[key] = recursively_enhance(val)
+          self[key] = Util.recursively_enhance(val)
         end
 
         freeze
@@ -82,22 +82,6 @@ module Middleman
         end
       end
 
-      private
-
-      Contract Any => Frozen[Any]
-      def recursively_enhance(data)
-        if data.is_a? HashWithIndifferentAccess
-          data
-        elsif data.is_a? Hash
-          self.class.new(data)
-        elsif data.is_a? Array
-          data.map(&method(:recursively_enhance)).freeze
-        elsif data.frozen? || data.nil? || [::TrueClass, ::FalseClass, ::Fixnum].include?(data.class)
-          data
-        else
-          data.dup.freeze
-        end
-      end
     end
   end
 end