Refactor code to create less objects

lib/mutant/cli/classifier.rb

@@ -63,12 +63,9 @@ module Mutant
       # @api private
       #
       def self.build(*arguments)
-        classifiers = REGISTRY.map do |descendant|
-          descendant.run(*arguments)
-        end.compact
-
+        classifiers = REGISTRY.map { |descendant| descendant.run(*arguments) }
+        classifiers.compact!
         raise if classifiers.length > 1
-
         classifiers.first
       end