Add flat_map -> map mutation

lib/mutant/mutator/node/send.rb

@@ -18,6 +18,7 @@ module Mutant
           reverse_each:  %i[each],
           reverse_merge: %i[merge],
           map:           %i[each],
+          flat_map:      %i[map],
           sample:        %i[first last],
           pop:           %i[last],
           shift:         %i[first],

meta/send.rb

@@ -128,6 +128,13 @@ Mutant::Meta::Example.add :send do
   mutation 'each'
 end
 
+Mutant::Meta::Example.add :send do
+  source 'flat_map'
+
+  singleton_mutations
+  mutation 'map'
+end
+
 Mutant::Meta::Example.add :send do
   source 'foo.to_s'