Minor printer cleanup for SMT-LIBv2 symbols "div" and "mod".

Thanks to Alex Horn for raising the issue on the CVC-BUGS mailing list.
Better user documentation for mkVar() and mkBoundVar().

Thanks to Alex Horn for raising the issue on the CVC-BUGS mailing list.
Remove macros EXPECT_TRUE / EXPECT_FALSE from cvc4_public.h so that they don't escape to user space

Thanks to Alex Horn for raising the issue on the CVC-BUGS mailing list.
Small fix for IS_INTEGER.
Remove output.h from public space, to avoid clashes with symbols defined in users' space.

Specifically, output.h was moved to CVC4's "private-library" rules, which means that it's
not installed on users' machines, and public headers should not include it.

Thanks to Alex Horn for raising the issue on the CVC-BUGS mailing list.
Fix minor warnings found by recent clang/gcc.