Internal implementation for RingDistrMPolyClean
The primary purpose for object of class DistrMPoly is to represent values in a RingDistrMPoly; consequently all operations on a DistrMPoly which could allocate or deallocate memory must take arguments indicating which memory manager(s) to use.
An object of type DistrMPoly is essentially an ordered (singly) linked list of summands, and each summand is a pair made up of a coefficient (held as a RingElem) and a power product (held as a PPMonoidElem). In a valid DistrMPoly all the coefficients are non-zero, the power products are all distinct and the summands are ordered decreasingly according to the ordering on the PPs (as determined by the PPO inside the owning PPMonoid).