Add runtime test feature

[soutaro]

This PR adds runtime test feature for RBS signatures, which validates all method calls of given classes and checks if the arguments and return values #is_a? specified types.

We assume you run the runtime test with tests or specs:

$ RUBYOPT=-rruby/signature/test/setup RBS_TEST_TARGET="Kaigi::*" bundle exec rake test

Discussions

• Generic types? => no test on generic parameters yet!
• Interface types? => no test yet! (I think using #respond_to? would make some sense.)
• Parametric polymorphism => no test yet! (We would be able to implement type inference, but not yet.)