Reduce memory per shm string by 8 bytes

[TysonAndre]

Related to #323

This slightly helps performance of apc.serializer=default when there are lots of small strings to unpersist. (and reduces size of serialized data (and cache entry keys) by 8 bytes with other serializers (16 bytes per entry))

With APCu's current design and eviction always being a possibility, strings must always be copied.
(zval flags could be used to check the type if that stopped being the case)

For example, fetching a large array mapping small strings to small strings (CPU running at 2.5GHz):

Before: bench_apcu_fetch_large_config: numValues=     65536
repetitions=       256 elapsed=3.010 shared memory=   9381144
After: bench_apcu_fetch_large_config: numValues=     65536
repetitions=       256 elapsed=2.980 shared memory=   8332560

Benchmark source code (click to expand)

<?php // bench_fetch_large_config.php

printf(
    "%s Results for php %s debug=%s with opcache enabled=%s, apc.serializer=%s\n\n",
    basename(__FILE__),
    PHP_VERSION,
    json_encode(PHP_DEBUG),
    json_encode(function_exists('opcache_get_status') && (opcache_get_status(false)['opcache_enabled'] ?? false)),
    ini_get('apc.serializer')
);

function bench_apcu_fetch_large_config(int $numValues, int $repetitions) {
    $values = [];
    srand(1234);
    apcu_clear_cache();
    for ($i = 0; $i < $numValues; $i++) {
        $values["key" . rand(0, PHP_INT_MAX)] = "value" . rand(0, PHP_INT_MAX);
    }
    if (!apcu_store('config_key', $values)) {
        throw new Exception("Failed to set config_key");
    }
    $expected = count($values);
    $start = hrtime(true);
    for ($i = 0; $i < $repetitions; $i++) {
        $v = apcu_fetch('config_key');
        if (count($v) !== $expected) {
            throw new RuntimeException("Failed to load config");
        }
    }
    $end = hrtime(true);
    printf("%s: numValues=%10d repetitions=%10d elapsed=%.3f shared memory=%10d\n", __FUNCTION__, $numValues, $repetitions, ($end - $start)/1e9, (int)apcu_cache_info()['mem_size']);
}

$settings = [
    [2**16, 2**8],
    [2**12, 2**12],
    [2**8,  2**16],
    [2**3,  2**21],
    [1,     2**22],
];
for ($i = 0; $i < 2; $i++) {
    foreach ($settings as [$n, $repetitions]) {
        bench_apcu_fetch_large_config($n, $repetitions);
    }
}