Reduce memory usage of SplObjectStorage

[TysonAndre]

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

<?php
call_user_func(function () {
    $startMem = memory_get_usage();
    $count = 1000000;
    $start = hrtime(true);
    $s = new SplObjectStorage();
    for ($i = 0; $i < $count; $i++) {
        $s->attach(new stdClass(), $i);
    }
    $end = hrtime(true);
    $endMem = memory_get_usage();
    // Before: 0.119 seconds 113.939 MiB (due to change from zval to pointer being stored)
    // After : 0.094 seconds 105.939 MiB (due to change to Z_PARAM_* to parse zend_object* directly)
    printf("Time to add %d objects to SplObjectStorage: %.3f mem=%.3fMiB\n", $count, ($end - $start)/1e9, ($endMem - $startMem) / 1000000);
});

[nikic]

Nice!