[patch 3/N] std::regex refactoring - _Executor DFS / BFS

[Jonathan Wakely]

I thought I'd make a 5x speedup to the run-time of the regex matching,
but I was comparing the wrong version and the improvement actually
came from one of your patches yesterday - maybe this one:
http://gcc.gnu.org/ml/gcc-patches/2014-04/msg01725.html

Nice work!

My changes don't seem to make anything worse, and the first one makes
a big improvement to the worst case performance of std::wregex.