Implement modular multivariate GCD (based on chinese remaindering algorithm).

Implement modular multivariate GCD (based on chinese remaindering algorithm).

Both algorithm and implementation are a bit inefficient:

* algorithm does not make use of sparseness of inputs (and most of
  multivariate polynomials are quite sparse)
* GiNaC's expressions are essentially immutable. Thus some simple
  operations (i.e. multiplying a polynomial by an element of the base ring)
  are prohibitively expensive.
* All numbers (i.e. GiNaC::numeric objects) are heap allocated.

Still it's much faster (~5x on bivariate polynomials, ~100x on 3-variate
ones) than (subresultant) PRS algorithm, so gcd() uses modular algorithm
by default.