Renaming of SMT2 operator names, kinds for set theory

* SET_SINGLETON kind renamed to just SINGLETON
* "setenum" smt2 opertor renamed to "singleton"[1]
* "in" smt2 operator renamed to "member"[2]

[1] It was anyhow accepting exactly one argument, so was bit misleading
    to call set enumerator.

[2] The corresponding kind was called MEMBER, so this will also make them
    consistent. Only inconsistency now is for subset: kind is called
    SUBSET but operator is called "subseteq".
make emptyset construction with no arguments private