Refactor internal calc_bound() routines.

Refactor internal calc_bound() routines.

Reduce them to computation of absolute values of the polynomial's roots
and document that this is known as Landau's inequality. The multiplication
with a power of two is done by the caller.

Also remove the internal fallback computation using the maxcoeff since
this cannot happen. (In the worst case, there is only one non-zero
coefficient 'a'. Then we compute sqrt(a^2) which is not smaller than a.
Otherwise, we compute sqrt(a^2 + x) > a.)