roundtripper: support generating then verifing the roundtripped data.
roundtripper: support generating then verifing the roundtripped data.

Interop test with Java, with keys coming from JS.

WeAreWizards/keyczarjs,schatt/keyczarjs,schatt/keyczarjs,schatt/keyczarjs,mitro-co/keyczarjs,mitro-co/keyczarjs,WeAreWizards/keyczarjs,mitro-co/keyczarjs,WeAreWizards/keyczarjs