Implement and test System.Rational.{+,-,*,/,%,to_string}
Implement and test System.Rational.==
Implement Rip::Core::Rational.from_native
Implement appropriate overloads for System.Integer and System.Rational to interoperate with each other

See comments at https://trello.com/c/SnazXMxo
FIXME Failing tests with `rspec --seed 56915`