lib: reintroduce v8 module

I introduced this module over a year ago in a pull request as the v8
module but it was quickly subsumed by the tracing module.

The tracing module was recently removed again and that is why this
commit introduces the v8 module again, including the new features it
picked up commits d23ac0e and f8076c4.

R=@trevnorris
Decision time: remove the documentation for the 'gc' event or document that it's dangerous?  I'm leaning toward the former.  That would let us remove the gc machinery from src/.  I don't need it for strong-agent.