Convert the assertions in std_expr.h to invariants as a demonstration.

Many of these are split into two invariants are there is the specific
precondition (before calling to_div_expr it is your responsibility to
check you have an exprt with id() == ID_div) and a general data structure
invariant (all ID_div exprts should have exactly two operands).

A few invariants are commented out as I believe they hold but they are
not currently enforced.  This commit should not change the functionality
of the code.

Author: martin