Performance: Precompile the ImmutableComponentTrait::encode() regex

[rbayliss]

This pattern is used in several places:

$reservedChars = implode('', array_map(function ($value) {
  return preg_quote($value, '/');
}, static::$characters_set));

preg_replace_callback( '/(?:[^'.$reservedChars.']+|%(?![A-Fa-f0-9]{2}))/' ...

This regex is being recalculated and rebuilt every time ImmutableComponentTrait::encode(), Path::validate(), or HierarchicalPath::validate() is being called. Believe it or not, compiling this regex accounts for ~50% of the time spent in this benchmark. This PR compiles the entire regex 1x per implementor of ImmutableComponentTrait. A blackfire run of that benchmark script will show anywhere from a 40-60% reduction in time spent running that script (blackfire run --samples=5 php perf.php). So basically, we get a huge speed boost with no downside that I can see.