Improve performance of unpack() with nameless repetitions

[nielsdos]

We can avoid creating temporary strings, and then reparsing them into numbers with zend_symtable_update() by using zend_hash_index_update() directly.

For the following benchmark on an i7-4790:

$file = str_repeat('A', 100000);
for ($i=0;$i<100;$i++) unpack('C*',$file);

I get:

Benchmark 1: ./sapi/cli/php test.php
  Time (mean ± σ):      85.8 ms ±   1.8 ms    [User: 74.5 ms, System: 10.4 ms]
  Range (min … max):    83.8 ms …  92.4 ms    33 runs

Benchmark 2: ./sapi/cli/php_old test.php
  Time (mean ± σ):     318.3 ms ±   2.7 ms    [User: 306.7 ms, System: 9.9 ms]
  Range (min … max):   314.9 ms … 321.6 ms    10 runs

Summary
  ./sapi/cli/php test.php ran
    3.71 ± 0.08 times faster than ./sapi/cli/php_old test.php

On an i7-1185G7 I get:

Benchmark 1: ./sapi/cli/php test.php
  Time (mean ± σ):      60.1 ms ±   0.7 ms    [User: 47.8 ms, System: 12.0 ms]
  Range (min … max):    59.2 ms …  63.8 ms    48 runs

  Warning: Statistical outliers were detected. Consider re-running this benchmark on a quiet system without any interferences from other programs. It might help to use the '--warmup' or '--prepare' options.

Benchmark 2: ./sapi/cli/php_old test.php
  Time (mean ± σ):     220.8 ms ±   2.2 ms    [User: 209.6 ms, System: 10.7 ms]
  Range (min … max):   218.5 ms … 224.5 ms    13 runs

Summary
  ./sapi/cli/php test.php  ran
    3.67 ± 0.06 times faster than ./sapi/cli/php_old test.php

[staabm]

nice to see the benchmark was useful. thanks for looking into it. 🙏

[nielsdos]

Looks like there's a missing null check somewhere, I'll look tomorrow or this evening.

[Girgias]

One minor suggestion, but this looks good to me.