Reduce memory usage of SplObjectStorage

Track an 8-byte pointer as the key instead of 16-byte zval.
(4 byte on 32-bit builds)

The memory used for a set of 1 million objects decreased from
114 MiB to 106 MiB.

Use the new `Z_PARAM_*` APIs to parse the `zend_object*` pointer directly

Author: TysonAndre