Specialize ccode() of Pow for integer exponents.

See also #7517

In C pow(my_float, 2) is not efficient (c.f. e.g. https://github.com/tjhladish/EpiFire/blob/master/gsl_subset/gsl/gsl_pow_int.h). This PR prints ccode of Pow instances as x*x*x*... for integer exponents less than 10.

Note that the possibility of users to override Pow which was enabled in #7517, has not changed.
Wouldn't the common subexpression optimizations make repeated multiplication effectively equivalent to exponentiation by squaring? If so, then I would expect it to be more efficient the larger the exponent. We should benchmark it to see, though.