Add expression => self mutation

Also reorganizes parts of meta to align filenames to node name better.

Closes #191

meta/blockarg.rb

@@ -3,8 +3,8 @@
 Mutant::Meta::Example.add do
   source 'foo { |&bar| }'
 
+  singleton_mutations
   mutation 'foo { |&bar| raise }'
   mutation 'foo {}'
   mutation 'foo'
-  mutation 'nil'
 end