Implement Enumerator::Lazy#filter.

Author: nirvdrum

spec/tags/core/enumerator/lazy/filter_tags.txt

@@ -379,6 +379,7 @@ def select
       end
     end
     alias_method :find_all, :select
+    alias_method :filter, :select
 
     def reject
       raise ArgumentError, 'Lazy#reject requires a block' unless block_given?