Improve performance of array_walk()

[ndossche]

We never need to use refcounted copies for arguments because the copy to
the call frame already increments the refcount.

For the following benchmark:

$a = range(0, 10);
for ($i = 0; $i < 1000000; $i++)
    array_walk($a, fn ($val, $idx, $arg) => $val + $arg, 2);

On an i7-4790:

Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):     593.0 ms ±   5.4 ms    [User: 589.8 ms, System: 1.7 ms]
  Range (min … max):   583.3 ms … 600.9 ms    10 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     637.8 ms ±   4.6 ms    [User: 633.9 ms, System: 2.2 ms]
  Range (min … max):   633.4 ms … 649.2 ms    10 runs

Summary
  ./sapi/cli/php x.php ran
    1.08 ± 0.01 times faster than ./sapi/cli/php_old x.php

On an i7-1185G7:

Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):     362.3 ms ±   2.0 ms    [User: 359.9 ms, System: 1.9 ms]
  Range (min … max):   359.6 ms … 367.1 ms    10 runs

Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     385.5 ms ±   1.8 ms    [User: 383.2 ms, System: 1.9 ms]
  Range (min … max):   381.5 ms … 387.2 ms    10 runs

Summary
  ./sapi/cli/php x.php ran
    1.06 ± 0.01 times faster than ./sapi/cli/php_old x.php