compute all optimal factorisations

[jannikmi]

add new function (feature) to compute all optimal horner factorisations of a given polynomial.

The existing A* search has to be adapted to not terminate once the first full factorisation has been found, but instead collect all fully factorized optimal solutions until the next best factorisation in the heap has a higher cost estimate

Attention: do not remove an entire factorisation subtree, but just one realisation each time!

For common polynomials the optimal factorisations can be precomputed and stored. This data can perhaps also be used to learn better heuristics for factorizing more optimally in less time.