Bug fix for issue #196 (#211)

Fixes #196.

Correct treatment of missing coefficients in `cnexp` solver.

* Extend `EXPECT_EXPR_EQ` functionality with wrapper that works with `Expression *` and `expression_ptr` arguments.
* Replace string comparison checks in `test_symdiff.cpp` with equivalents that use `EXPECT_EXPR_EQ`.
* Check explicitly for missing coefficient in `cnexp` solver, which should be treated equivalently to zero.