Fix series of positive powers of polynomials.

Fix series of positive powers of polynomials.

Before calling pseries::power_const(), make sure the basis series has
sufficient terms. In particular, do not shrink the order of expansion
- only grow it.

Fix various hairy problems in pseries::power_const() when a polynomial
is raised to a positive integer power.

(The special cases here can actually also be computed by Taylor
expansion but the fixes should be more general.)

Also add test cases.

Thanks to Vitaly Magerya <vmagerya@gmail.com> for reporting this.