Use WeakMap and abolish CastedCData

[sj-i]

The use of CastedCData is to keep the refcount of \FFI\CData objects.

However, this solution is clearly ugly and causes confusing use-after-free.

Another solution is using \WeakMap. If we use partial \FFI\CData objects as keys to their root \FFI\CData object in a \WeakMap, we can guarantee that the root object will survive while at least one reference count of the partial objects remains.

There was a bug in php-src that prevented \WeakMap from being used with FFI extension objects, but a fix has already been released, so it's time to move to \WeakMap.