Use CLoader for improved YAML parsing speed.

The default YAML parser is very slow for large
files. The CLoader takes advantage of an optimized
C version and is significantly faster.

Note that while this patch somewhat alleviates the
pain with current rospack/rosdep performance,
it is still noticeably slow.

src/rosdep2/sources_list.py

@@ -145,7 +145,7 @@ def create_model(type_, uri, tags, origin=None):
             if verbose:
                 print("loading cached data source:\n\t%s\n\t%s"%(uri, filepath), file=sys.stderr)
             with open(filepath) as f:
-                rosdep_data = yaml.load(f.read())
+                rosdep_data = yaml.load(f.read(), Loader=yaml.CLoader)
         else:
             rosdep_data = {}
         return CachedDataSource(type_, uri, tags, rosdep_data, origin=filepath)