src: remove ObjectWrap dependency from core

Drop the ObjectWrap dependency in favor of an internal WeakObject class.

Let's us stop worrying about API and ABI compatibility when making
changes to the way node.js deals with weakly persistent handles
internally.
v8: upgrade to 3.20.17.13

fixes #6235
test: add regression test for #6235
fs: remove duplicate !options case

I haven't actually tested this code, but was reading it due to a
post that linked to the code here:

    http://dailyjs.com/2013/09/26/libuv/

As I was reading through the code, I noticed a path that can't
be reached.

I didn't strictly follow the contributing guide:

    https://github.com/joyent/node/wiki/Contributing

but the change seems safe.

Feel free to close this out. I'm not sure if it was just an oversight
or what.
debugger: repeat last command

Regression introduced by commit 9ef9a9d.
debugger: show current line, fix for #6150
test: update require path after file move

Commit 204228b moved a few slow tests to pummel but I forgot to update
the require() path in pummel/test-debugger-repl-break-in-module.  Mea
culpa.