Code paths for *

This polynomial shows that multiplication for different polynomial types can vary significantly:

```
julia> rts = [1.0, sqrt(2), sqrt(3)];

julia> ls = [2, 3, 4];

julia> x, xx = variable(Polynomial{Float64}), variable(ImmutablePolynomial{Float64});

julia> pp = prod((xx-z)^l for (z,l) in zip(rts, ls));

julia> p = prod((x-z)^l for (z,l) in zip(rts, ls));

julia> maximum(abs, coeffs(pp*pp) .- coeffs(p*p))
2.3283064365386963e-10
```