Support accessing built-in filters as callbacks

[clue]

Marking this as WIP until we address flushing/ending the filter (#4). Edit: done.

Closes #3

[clue]

Upon further inspection of the above build errors it turns out this is caused by incompatibilities (aka bugs) in PHP 7 and HHVM respectively:

• HHVM inconsistency
  • Using DEFLATE (compression) algorithm returns strings that are bigger than the input string
  • Minimal reproducible script: https://3v4l.org/eK3Xa
  • Though this works fine: https://3v4l.org/jfDem
  • FWIW: HHVM buffers the whole string anyway, so I'm not sure how useful it's going to be anyway: https://github.com/facebook/hhvm/blob/master/hphp/system/php/stream/default-filters.php#L77
• PHP7 inconsistency
  • Using DEFLATE (compression) on an empty string should return a 2 byte output string
  • Minimal reproducible script: https://3v4l.org/hpsJA
  • Though this works fine: https://3v4l.org/A0teN

There's not really much we can do about this here.

As such, I'll look into documenting this as known issues and skip the relevant tests here.