Use SWAR to seek for non-ASCII UTF-8 in DOM parsing

[nielsdos]

GitHub FYP test case:

Benchmark 1: ./sapi/cli/php test.php
  Time (mean ± σ):     502.8 ms ±   6.2 ms    [User: 498.3 ms, System: 3.2 ms]
  Range (min … max):   495.2 ms … 509.8 ms    10 runs

Benchmark 2: ./sapi/cli/php_old test.php
  Time (mean ± σ):     518.4 ms ±   4.3 ms    [User: 513.9 ms, System: 3.2 ms]
  Range (min … max):   511.5 ms … 525.5 ms    10 runs

Summary
  ./sapi/cli/php test.php ran
    1.03 ± 0.02 times faster than ./sapi/cli/php_old test.php

Wikipedia English homepage test case:

Benchmark 1: ./sapi/cli/php test.php
  Time (mean ± σ):     301.1 ms ±   4.2 ms    [User: 295.5 ms, System: 4.8 ms]
  Range (min … max):   296.3 ms … 308.8 ms    10 runs

Benchmark 2: ./sapi/cli/php_old test.php
  Time (mean ± σ):     308.2 ms ±   1.7 ms    [User: 304.6 ms, System: 2.9 ms]
  Range (min … max):   306.9 ms … 312.8 ms    10 runs

Summary
  ./sapi/cli/php test.php ran
    1.02 ± 0.02 times faster than ./sapi/cli/php_old test.php

[Girgias]

Looks sensible