add MapKey class to Util.YAML
add MapKey class to Util.YAML

Ignore-this: 60a6ec7559d5e3301b97453664ea0a1f

darcs-hash:1a7714d95819a6f2bcde1e214e484cdd22969603

dpwright/jhc,dpwright/jhc,dpwright/jhc,dpwright/jhc