- Kills set of warnings caused by a redundant `let(:output)`
in cli_spec.rb
- Kills warnings generated by the `with` class method that
spec/support/shared_context.rb provides by using `Module#prepend`
* Before Mutant::Expression was not marshallable resulting in the need
to build "partial" result objects from killforks.
* Fixes an adjacent bug in spec selection
* Flattens the rspec example groups on filtering to remove redundant
work being done on executing subtrees of already executed example
groups
* Uses test batching to kill one mutation per isolation that results in
a significant speedup.
* Drop rspec 2 support.
[closes#256]
