Support for having two SmtEngines with the same ExprManager.

Support for having two SmtEngines with the same ExprManager.

Basically, this involves creating a separate StatisticsRegistry for the
ExprManager and for the SmtEngine.  Otherwise, theories register the
same statistic twice.  This is a larger problem, though, for creating
multiple instances of theories, and that is unaddressed.  Still,
separating out the expr statistics into a separate registry is probably
a good idea, since the expr package is somewhat separate anyway (and in
the short term it allows two SmtEngines to co-exist).