Pass back V8Object instances, don't re-wrap

[stesie]

Currently if you pass a JS object to PHP it's wrapped to a V8Object instance.
If you pass this V8Object instance back to JavaScript it's wrapped again, i.e. a different JS object is constructed. That is

• inconsistent with V8Function behaviour (where we already skip re-wrapping)
• hurts object identity
• doesn't work at all if the object is an ES6 Promise