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/begin.rb

@@ -0,0 +1,15 @@
+# encoding: utf-8
+
+Mutant::Meta::Example.add do
+
+  source 'true; false'
+  # Mutation of each statement in block
+  mutation 'true; true'
+  mutation 'false; false'
+  mutation 'nil; false'
+  mutation 'true; nil'
+
+  # Delete each statement
+  mutation 'true'
+  mutation 'false'
+end