src: preliminary update to new Persistent API

Large changes are coming down the pipe from v8 for how Persistents are
handled. These changes are preparing node for these.

This is still a WIP, and actually not completely sure these are the absolute correct syntax changes. Just going by the `// TODO(dcarney): remove before cutover` comments found in `include/v8.h`.

/cc @bnoordhuis @isaacs