[Yaml-core] PyYaml profiling

[Steve H. <sh...@zi...>]

I played around with Python's profiler today, and there were some easy
optimizations for PyYaml.  The next release should load documents roughly 30%
faster.  The easiest speedups were to use compiled regexes, and I also found a
loop where I was needlessly checking the same line twice to see if it was a
comment.

It may be a little while before I ship the next release, though, because I'm
about to hit the road for a few days.