Dereference values to always get the underlying value

[crocodele]

Fixes #150, based on info in https://www.phpinternalsbook.com/php7/zvals/references.html.

[mk6i]

@crocodele Nice find!

I was a bit curious as to why there were two calls to ZVAL_DEREF() and dug into the code a bit. Turns out it's a smell for a refactoring I've been meaning to do. Previously, we passed tok->next() to exec_recursive_descent(). To be consistent with how the other functions work, we should pass the actual AST_RECURSE token instead. Now we should only need to call ZVAL_DEREF() in one place.

Would you please take a look at my last commit? Feel free to merge this PR if you approve.

[crocodele]

@mkaminski1988, thanks, makes sense. 🙂