Define expected mutations with less redundancy

The mutation examples are now introspectable, this allows to render nice
index or performing better automated analysis against the corpus.

meta/block_pass.rb

@@ -0,0 +1,8 @@
+# encoding: utf-8
+
+Mutant::Meta::Example.add do
+  source 'foo(&bar)'
+
+  mutation 'foo'
+  mutation 'nil'
+end