Speed up yaml output by using CDumper

The libyaml implementation is significantly faster than the pure python
YAML parser. If it is present, then use CDumper when dumping the
results.
Signed-off-by: Grant Likely <grant.likely@arm.com>
[Rebased, added conditional.]
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>

parser.py

@@ -13,6 +13,12 @@ try:
 except ImportError:
     print('No yaml...')
 
+if 'yaml' in sys.modules:
+    try:
+        from yaml import CDumper as Dumper
+    except ImportError:
+        from yaml import Dumper
+
 
 #based loosley on https://stackoverflow.com/a/4391978
 # returns a filtered dict of dicts that meet some Key-value pair.
@@ -343,7 +349,7 @@ def gen_yaml(cross_check, filename):
     logging.debug(f'Generate {filename}')
 
     with open(filename, 'w') as yamlfile:
-        yaml.dump(cross_check, yamlfile)
+        yaml.dump(cross_check, yamlfile, Dumper=Dumper)
 
 
 # Combine or two databases db1 and db2 coming from ekl and seq files