Use yaml.CSafeLoader whenever possible for speedups

[wouterzwerink]

Is your feature request related to a problem? Please describe.
yaml.CSafeLoader is much faster than the currently used yaml.SafeLoader

Describe the solution you'd like
Try to use yaml.CSafeLoader (not always possible) and fall back to the old loader.

Describe alternatives you've considered
Keeping the slower parser or monkey patching this externally instead of supporting it in OmegaConf itself.
Allowing the user to set a base loader could also work