Don't allow get-model & co after a user push/pop

This makes us more strictly adhere to the spec, but it's useful anyway:
previously we would support a get-model until the problem was explicitly
changed with e.g. a new assertion.  That meant you could check-sat, then
pop, then get-model, but you'd only get the part of the model still in
scope.  This is strange, and would likely lead to problems, so it's now
disabled.

Thanks to David Cok for inquiring about this.
Compliance fixes for :named annotations: they must name closed subterms, the names must be user-permitted names, etc.

Thanks to David Cok for raising this issue.
Don't allow --stats if not a statistics-enabled build
fix arithmetic error reporting on use of (nonlinear) div,mod,/ symbols