RecursiveRegexIterator's children don't use their parents flags

Summary: I don't think this is the right behaviour, but it matches php...

Also apply patch to bug68128 (Array to String conversion in accept())

The slow test I added tests for the children's flags. PHP reports NULL for pregFlags whereas we report 0.
I reported this at https://bugs.php.net/bug.php?id=68175

Fixes #3909

Closes https://github.com/facebook/hhvm/pull/3915

Reviewed By: @paulbiss

Differential Revision: D1599880
Add benchy.py for benchmarking HHVM

Summary: Adds benchy.py and all of its supporting scripts. benchy.py is
a benchmarking harness that handles checkout out, building,
benchmarking, and analyzing performance for HHVM.

Reviewed By: @jasone

Differential Revision: D1615647