Fix example of symmetrization in tutorial.

The old example was illegal C++. Thanks to Rubel Zilibowitz for
reporting this error.
[bugfix] ex::is_polynomial(x): fix miscomputations

The function power::is_polynomial(x) sometimes returned false,
even if x did not occur in the power object.

Thanks to Rubel Zilibowitz for reporting this bug.

Similarly, mul::is_polynomial(x) sometimes returned true, even
if x occurred only in the denominator of a rational function.

The function mul::is_polynomial(x) does a simpler version of
power::is_polynomial(x) on its factors. (It's simpler because the
exponents cannot be functions of x.)
Providing default (un)archiving for cliffordunit class.

Signed-off-by: Vladimir V. Kisil <kisilv@maths.leeds.ac.uk>
Add more ex::is_polynomial() tests

Improves commit 92da4c16 ('[bugfix] ex::is_polynomial(x): fix miscomputations')
a little bit.