expose mpz_poly interface in two different ways.

Some functions in the mpz_poly API can optionally use openmp. We want to make sure that the code that calls these openmp-enabled functions does so willingly. And we don't want to pollute the "normal" interface.

The chosen solution is as follows.

• the functions declared in mpz_poly.h, and used as plain (e.g.) mpz_poly_mul resolve to something that does NOT use openmp.

• to use openmp, include mpz_poly_parallel.hpp instead (which duplicates some of the declarations of mpz_poly.h), and call the member functions of an mpz_poly_parallel_info object. E.g.

  mpz_poly_parallel_info inf;
  // (maybe add some configuration code for the inf object, if the
  // need for that ever appears)
  inf.mpz_poly_mul(....)