Use HOLset for storing free vars in the Traversal Engine

[ordinarymath]

Right now that I've found that free_varsl in Traverse.add_context(2)doit(2) is a hot path the simplifier. One issue is that variant called in EQ_CONGPROC is can show different performance on the order of the list it's passed in. The proper solution to that would be #1642. A workaround for this issue could also be seeing if using FVL followed by using HOLset.listItems works as a workaround.