Add a 1-char fastpath to implode()

[alexandre-daubois]

I think it's worth adding a 1 char fast path for implode(). A quick search on Github reveals the following stats:

• Implode usage with one char glue: /implode\(\s*["'][^"']{1}["']\s*,\s*/ language:PHP reveals 1.4 million occurrences
• Implode usage with 2+ char glue: /implode\(\s*["'][^"']{2,}["']\s*,\s*/ language:PHP reveals 1.1 million occurrences

Here is the benchmark code:

<?php

$iterations = 1000;
$array = array_fill(0, 10000, 'item');

for ($i = 0; $i < $iterations; $i++) {
    implode(',', $array);
}

And the results:

alex@alex-macos php-src % hyperfine './sapi/cli/php.branch validation_benchmark.php' './sapi/cli/php.master validation_benchmark.php' --warmup 50
Benchmark 1: ./sapi/cli/php.branch validation_benchmark.php
  Time (mean ± σ):     313.4 ms ±   4.7 ms    [User: 309.0 ms, System: 2.8 ms]
  Range (min … max):   307.9 ms … 321.5 ms    10 runs
 
Benchmark 2: ./sapi/cli/php.master validation_benchmark.php
  Time (mean ± σ):     419.6 ms ±   6.9 ms    [User: 411.9 ms, System: 4.0 ms]
  Range (min … max):   412.4 ms … 435.6 ms    10 runs
 
Summary
  ./sapi/cli/php.branch validation_benchmark.php ran
    1.34 ± 0.03 times faster than ./sapi/cli/php.master validation_benchmark.php

No change for multi char glue, but a nice improved performance for single char glue especially one big arrays.