Replace SExpr class by simpler conversion routines (#6363)

Replace SExpr class by simpler conversion routines (#6363)

This PR finally removes the SExpr class. SMT-LIB compatible output is retained by using new on-the-fly conversion to s-expression strings. This finally allows us to remove includes to integer and rational from smt_engine.h.
In detail:

- a new set of toSExpr() methods is implemented that converts certain types to s-expression strings (without an intermediate class representing s-expressions)
- SmtEngine::getInfo() returns a string instead of SExpr and uses the new toSExpr methods
- SmtEngine::getStatistic() is removed
- SExpr class is removed
- d_commandVerbosity uses int instead of Integer