Require mutant-rspec on rspec integration specs
Fix invalid AST emit

* Mutant prevents double emits via guarding on source representation of
  partial AST nodes. Not all AST nodes are valid as roots, so this
  change creates a valid root around an leave.

* Over the time multiple of these situartions must be solved, we'll end
  up in a more deduplicated solution.

* Closes: #165
Remove dead code