fix a bug in _hensel_lift
fix typo in _leading_coeffs
include the defect of the algebraic field in some computations
remove unnecessary if statement in PolyElement.evaluate
fix computations of leading coefficients
cover cases where the leading coefficient is 'unlucky'

When two irrducible factors of `lc_{x_0}(f)` have the same norm, no good
evaluation points can be found. That case is now covered.
cover cases where the minimal polynomial is 'unlucky'

Some minimal polynomials are reducible modulo p for any prime p. In that
case, the factorization has to be computed with dmp_ext_ factor.
choose a new prime if Hensel lift failed
add tests for the factorization of polynomials over algebraic number fields