fix https://github.com/sagemathinc/smc/issues/1710  -- older browsers do not know "Set()" -- fix by rewriting to use Javascript map.
fix https://github.com/sagemathinc/smc/issues/1710  -- older browsers do not know "Set()" -- fix by rewriting to use Javascript map.

tscholl2/smc,tscholl2/smc,sagemathinc/smc,sagemathinc/smc,sagemathinc/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,DrXyzzy/smc,DrXyzzy/smc,DrXyzzy/smc,tscholl2/smc,sagemathinc/smc